780mpa grade rare earth high-strength dual-phase steel and preparation method and application thereof

Abstract

This invention discloses a 780MPa-grade rare-earth high-strength dual-phase steel, its preparation method, and its applications. Belonging to the technical field of rare-earth dual-phase steel, the chemical composition and mass percentage of the 780MPa-grade rare-earth high-strength dual-phase steel are as follows: C: 0.08-0.10%; Si: 0.15-0.25%; Mn: 1.8-2.4%; Als: 0.020-0.055%; Nb: 0.025-0.035%; Ti: 0.015-0.025%; Ce: 0.0026-0.0045%; P: ≤0.012%; S: ≤0.004%; N: ≤0.004%, with the remainder being Fe and unavoidable impurities. In this invention, the proportion of large-angle grain boundaries in the rare-earth high-strength dual-phase steel is 49.4%-56.4%. From the microstructure and crystal orientation results, the rare-earth steel of this invention exhibits better toughness and plastic deformation capacity.

Description

Technical Field

[0001] This invention belongs to the field of rare earth dual-phase steel technology, and particularly relates to a 780MPa grade rare earth high-strength dual-phase steel, its preparation method and application. Background Technology

[0002] In rare earth high-strength dual-phase steel, increasing the proportion of large-angle grain boundaries is an important means to strengthen the material and improve its mechanical properties (such as strength, toughness, and resistance to crack propagation). Existing technologies include methods for increasing the proportion of large-angle grain boundaries: Rare earth element addition: Rare earth elements (such as Ce, La, Y, etc.) can effectively refine grains, purify grain boundaries, and inhibit the growth of original austenite grains at high temperatures, thereby promoting the formation of more large-angle grain boundaries.

[0003] Controlled heat treatment process: Critical zone annealing or two-phase zone heat treatment can promote the phase transformation of ferrite to martensite / austenite and increase grain boundary density. Rapid cooling helps to suppress grain growth and retain more large-angle grain boundaries.

[0004] Plastic deformation + recrystallization control: High-density dislocations are introduced through cold rolling or warm deformation, followed by low-temperature annealing, which promotes dynamic recrystallization to generate fine equiaxed crystals and increases the proportion of large-angle grain boundaries.

[0005] Synergistic effect of microalloying: Combining microalloying elements such as Nb, Ti, and V, fine carbonitride precipitates are formed, which pin grain boundaries and inhibit grain growth, thereby maintaining a high proportion of large-angle grain boundaries.

[0006] Optimize casting and solidification conditions: Using electromagnetic stirring, ultrasonic vibration or rapid solidification technology can refine the as-cast grains, providing a finer original structure for subsequent processing, which is beneficial for obtaining a high proportion of large-angle grain boundaries in the end.

[0007] While the aforementioned methods can increase the proportion of large-angle grain boundaries, they present the problem of difficulty in achieving a synergistic effect between strength, plasticity, and toughness. An excessively high proportion of large-angle grain boundaries, although helpful in refining grains, may lead to uneven local plastic deformation if accompanied by high-density, shear-insensitive precipitates (such as nanoprecipitates), particularly resulting in poor transverse bending performance, and even causing a decrease in elongation or brittle fracture. Therefore, there is an urgent need for a 780MPa-grade rare-earth high-strength dual-phase steel that can solve the above technical problems, achieve a synergistic improvement in strength, plasticity, and toughness, good formability, and significantly enhanced transverse bending performance, along with its preparation method and applications. Summary of the Invention

[0008] The purpose of this invention is to provide a 780MPa grade rare earth high-strength dual-phase steel with a high proportion of large-angle grain boundaries, excellent material strength and toughness, and excellent formability.

[0009] Meanwhile, this invention provides a method for preparing 780MPa-level rare-earth high-strength dual-phase steel. In this method, the rare-earth components are added in two batches, significantly improving the banded structure at the 1 / 2 thickness position of the rare-earth high-strength dual-phase steel. The grains are refined, and the martensite distribution morphology is controlled, with a high proportion of large-angle grain boundaries. This effectively alters the crack propagation path, increases crack propagation resistance, and enhances the material's toughness and impact resistance. The material exhibits excellent strength and toughness, as well as excellent formability.

[0010] Meanwhile, this invention provides a 780MPa grade rare earth high-strength duplex steel for application in the automotive manufacturing field, suitable for complex bending and stamping automotive parts.

[0011] To achieve the above objectives, the present invention provides the following technical solution: A 780MPa grade rare earth high-strength dual-phase steel has the following chemical composition and mass percentage: C: 0.08-0.10%; Si: 0.15-0.25%; Mn: 1.8-2.4%; Als: 0.020-0.055%; Nb: 0.025-0.035%; Ti: 0.015-0.025%; Ce: 0.0026-0.0045%; P: ≤0.012%; S: ≤0.004%; N: ≤0.004%; with the remainder being Fe and unavoidable impurity elements.

[0012] A method for preparing 780MPa grade rare earth high-strength dual-phase steel includes the following steps: Step 1, Converter smelting: Scrap steel addition 30-50t, molten iron temperature 1240-1480℃, final temperature 1620-1660℃, add the following raw materials: electrolytic manganese (grades such as DJMn99.9, DJMn99.8, DJMn99.7, etc.) 2950-3000kg, medium carbon ferromanganese (grades such as FeMn78C2.0, FeMn75C2.0, etc.) 2 The amount of ferrosilicon manganese (grades such as FeMn64Si27, FeMn67Si23, FeMn68Si22, etc.) added is 3000-3050kg, and the amount of aluminum blocks (grades such as 1060, 1050, 1A97, etc., high-purity aluminum, purity ≥99.97%) added is 370-420kg. The converter smelting time is 60-100min, and the steel is tapped using a sliding plate to block slag.

[0013] Step 2, LF refining: Heat to 1610-1630℃, add 430-480kg of high-carbon ferromanganese (grades such as FeMn65C7.0, FeMn70C7.0, etc.), 100-120kg of ferroniobium (grades such as FeNb65, FeNb70, etc.), and 230-250kg of aluminum granules (grades such as 1060, 1050, 1A97, etc., high-purity aluminum, purity ≥99.97%), slag formation and desulfurization, LF refining time is 30-40min.

[0014] Step 3, RH refining: The molten steel exits the LF refining furnace and undergoes a three-stage RH refining process. First stage: Ultimate vacuum: 0-1.2 mbar, vacuum degassing, pure degassing time 5-12 min; Second stage: Add 180-190 kg of high-carbon ferromanganese (grades such as FeMn65C7.0, FeMn70C7.0, etc.), 70-85 kg of ferrotitanium (grades such as FeTi70), 15-20 kg of aluminum granules (grades such as 1060, 1050, 1A97, etc., high-purity aluminum, purity ≥99.97%), 90-110 kg of ferrosilicon (grades such as FeSi90, FeSi75, FeSi65, etc.), and 15-20 kg of CeFe alloy (grades such as CeFe-90, CeFe-95). The vacuum treatment time for this stage is 20-40 min, and the ultimate vacuum degree is 1.1-1.5 mbar. Third stage: Add 10-15 kg of CeFe alloy (grades such as CeFe-90, CeFe-95). Vacuum treatment time for this stage is 5-10 min, ultimate vacuum degree: 0.3-0.5 mbar; molten steel exit temperature is 1560-1570℃.

[0015] Step 4, continuous casting: During the continuous casting process, the constant casting speed is 1.1-1.2 m / min, the stopper rod rises slightly, and the liquid level fluctuation is controlled within ±5 mm to obtain the billet.

[0016] Step 5, hot rolling: The billet is hot rolled with an inlet temperature of 950-1000℃, a final rolling temperature of 890-910℃, a coiling temperature of 545-563℃, and a hot-rolled thickness of 2.5-4.1mm.

[0017] Step 6, cold rolling: pickling reduction rate 51%-55%, tension leveling rate 0.37-1.18%, rolling speed 256-412m / min, pickling speed 114-191m / min, finished product thickness 1.2-2.0mm.

[0018] Step 7, continuous annealing: In the annealing furnace, the process speed is 43-110 m / min, the temperature of the slow cooling section is 643-651℃, the temperature of the soaking section is 787-810℃, the temperature of the rapid cooling section is 261-305℃, and the flattening elongation is 49-51%.

[0019] The rare earth high-strength dual-phase steel Ce780 obtained by this invention has the following tensile strength range (tested from 22 groups of tensile samples): 793-817MPa, fluctuation range: 24MPa; yield strength range: 445-480MPa, fluctuation range: 35MPa; and elongation range: 20.2-26.1%.

[0020] Comparative example 780DP: Tensile strength range (tested 22 groups of tensile samples): 728-782MPa, fluctuation range: 54MPa; Yield strength range: 380-436MPa, fluctuation range: 56MPa; Elongation range: 12.0-15.0%.

[0021] This invention provides a 780MPa grade rare earth high-strength duplex steel for application in the automotive manufacturing field, suitable for complex bending and stamping automotive parts.

[0022] Applications in automotive components such as crossbeams, front longitudinal beams, and door sill reinforcements in passenger vehicle chassis.

[0023] An automotive component is prepared from a 780MPa grade rare-earth high-strength dual-phase steel according to this invention. The component is stamped under the following conditions: When the forming direction is transverse: the limiting sharp angle R ≥ 0.2mm, and the number of stamping internal angles ≤ 147 degrees; when the limiting sharp angle R ≥ 1.0mm, the number of stamping internal angles satisfies any angle forming from 0-180 degrees. When the forming direction is longitudinal: the limiting sharp angle R ≥ 0.2mm, and the number of stamping internal angles satisfies any angle forming from 0-180 degrees.

[0024] Compared with the prior art, the beneficial effects of the present invention are as follows: In this invention, the proportion of large-angle grain boundaries in Ce780 is 49.4%-56.4%, with an average of 52%. From the results of microstructure and crystal orientation, the rare earth steel of this invention has better toughness and plastic deformation capability.

[0025] The cold-rolled rare earth dual-phase steel of the present invention exhibits excellent forming performance. When bent longitudinally at 180 degrees with a bending radius of 0.2 mm (R=0.16t), it shows no cracks. Its transverse bending performance is significantly improved. Under bending radii of 0.2-0.8 mm (R=0.16t to R=0.67t), the ultimate bending angle of the rare earth Ce780 of the present invention is 9-13° greater than that of 780DP in Comparative Example 1. Attached Figure Description

[0026] The technical solution and experimental results of the present invention will be further described in detail below with reference to the accompanying drawings and microscopic tissue photographs.

[0027] Figure 1 The image shows the results of optical microscopy examination at the 1 / 2 thickness position of the hot-rolled state in Example 1; the upper image is magnified 500 times, and the lower image is magnified 1000 times. Figure 2 The image shows the results of optical microscopy examination at the 1 / 2 thickness position of the hot-rolled state in Comparative Example 1; the upper image is magnified 500 times, and the lower image is magnified 1000 times. Figure 3 Product images of rare earth dual-phase steels obtained in Example 1 and Comparative Example 1; Figure 4 A map showing the test locations for electron backscatter diffraction (EBSD) analysis; Figure 5 Electron backscatter diffraction (EBSD) layered image; Figure 6 for Figure 5 BC plot of electron backscatter diffraction (EBSD); Figure 7 for Figure 5 Euler angle color map of electron backscatter diffraction (EBSD); Figure 8 for Figure 5 Inverse pole figure of electron backscatter diffraction (EBSD) along the Z-axis; Figure 9 for Figure 5 Phase distribution diagram of electron backscatter diffraction (EBSD); Figure 10 Figure 5 Electron backscatter diffraction (EBSD) distribution diagram of grain boundary ratio at different angles; Figure 11 The image shows the EBSD plot of 780DP-A1 (the first position in the center layer of the 780DP sample) in Comparative Example 1. Figure 12 The results of the large-angle (≥15 degrees) grain boundary percentage detection for Example 1 and Comparative Example 1; Figure 13 This is a forming diagram of rare earth high-strength dual-phase steel in Example 1. Detailed Implementation

[0028] The present invention will now be described in further detail with reference to the accompanying drawings and specific embodiments. The following embodiments are for illustrative purposes only and are not intended to limit the scope of the invention.

[0029] Example 1: Abbreviated as Ce780

[0030] A 780MPa grade rare earth high-strength dual-phase steel has the following chemical composition and mass percentage: C: 0.088%; Si: 0.168%; Mn: 2.40%; Als: 0.026%; Nb: 0.027%; Ti: 0.019%; Ce: 0.0045%; P: 0.012%; S: 0.004%; N: 0.004%; with the remainder being Fe and unavoidable impurity elements.

[0031] A method for preparing 780MPa grade rare earth high-strength dual-phase steel includes the following steps: Step 1, converter smelting: 40t of scrap steel is added, the molten iron temperature is 1343℃, the final temperature is 1648℃, and the following raw materials are added: 2999kg of electrolytic manganese (grade DJMn99.9), 300kg of medium carbon ferromanganese (grade FeMn78C2.0), 3012kg of ferrosilicon manganese (grade FeMn64Si27), and 400kg of aluminum blocks (grade 1060, high purity aluminum, purity ≥99.97%). The converter smelting time is 80min, and the slag is blocked by a sliding plate when tapping the steel.

[0032] Step 2, LF refining: Heat to 1612℃, add 446kg of high-carbon ferromanganese (grade FeMn65C7.0), 110kg of ferroniobium (grade FeNb65), and 238kg of aluminum granules (grade 1060, high-purity aluminum, purity ≥99.97%), slag formation and desulfurization, LF refining time is 35min.

[0033] Step 3, RH refining: The molten steel exits the LF refining furnace and undergoes a three-stage RH refining process. First stage: Ultimate vacuum: 0.8 mbar, vacuum degassing, pure degassing time 10 min; Second stage: Add 182 kg of high-carbon ferromanganese (grade FeMn65C7.0), 79 kg of ferrotitanium (grade FeTi70), 18 kg of aluminum granules (grade 1060, high-purity aluminum, purity ≥99.97%), 99 kg of ferrosilicon (grade FeSi90), and 18 kg of CeFe alloy (grade CeFe-90). The vacuum treatment time for this stage is 30 min, and the ultimate vacuum degree is 1.3 mbar. Third stage: Add 12kg of CeFe alloy (grade CeFe-90), vacuum treatment time for this stage is 8min, ultimate vacuum degree: 0.4mbar; molten steel exit temperature is 1565℃.

[0034] Step 4, continuous casting: During the continuous casting process, the constant casting speed is 1.15m / min, the stopper rod rises slightly, and the liquid level fluctuation is controlled within ±5mm to obtain the billet.

[0035] Step 5, hot rolling: The billet is hot rolled with an entry temperature of 980℃, a final rolling temperature of 901℃, a coiling temperature of 548℃, a billet raw material thickness of 236mm, and a hot rolling thickness of 2.5mm.

[0036] Step 6, cold rolling: pickling reduction rate 52%, tension leveling rate 1.09%, rolling speed 333m / min, pickling speed 191m / min, finished product thickness 1.2mm.

[0037] Step 7, continuous annealing: thickness 1.2mm, width 1569mm, in the annealing furnace, the process speed is 81m / min, the slow cooling zone temperature is 644℃, the soaking zone temperature is 788℃, the rapid cooling zone temperature is 291℃, and the flattening elongation is 49.53%.

[0038] The rare-earth high-strength dual-phase steel obtained in this embodiment is applied to the automotive processing and manufacturing field. The automotive parts combine high strength and high formability, and are suitable for complex bending and stamping of automotive parts. Specifically, it is used in automotive parts such as the lower crossbeam, front longitudinal beam, and door sill reinforcement of passenger car chassis.

[0039] An automotive component is prepared from a 780MPa grade rare earth high-strength duplex steel according to this embodiment.

[0040] Comparative Example 1: Abbreviated as 780DP

[0041] The only difference between this comparative example and Example 1 is that: In the RH refining process, the CeFe alloy is fed in a single batch, specifically: First stage: Ultimate vacuum: 0.8 mbar, vacuum degassing, pure degassing time 10 min; Second stage: Add 182 kg of high-carbon ferromanganese (grade FeMn65C7.0), 79 kg of ferrotitanium (grade FeTi70), 18 kg of aluminum granules (grade 1060, high-purity aluminum, purity ≥99.97%), 99 kg of ferrosilicon (grade FeSi90), and 30 kg of CeFe alloy (grade CeFe-90). The vacuum treatment time for this stage is 30 min, and the ultimate vacuum degree is 1.3 mbar. Third stage: Vacuum treatment time in this stage is 8 minutes, ultimate vacuum degree: 0.4 mbar; molten iron exit temperature is 1565℃.

[0042] like Figure 1 The image shows the results of optical microscopy examination at the 1 / 2 thickness position of the hot-rolled steel in Example 1. It can be seen that the strip structure at the 1 / 2 thickness position of the 780MPa grade rare earth high-strength dual-phase steel obtained in Example 1 is significantly improved. Moreover, compared with Comparative Example 1, the lath martensite grains in Example 1 are refined and the martensite distribution morphology is controlled.

[0043] Figure 2 The results of optical microscopy examination at the 1 / 2 thickness position of the hot-rolled state of Comparative Example 1 show that the 780DP obtained in Comparative Example 1 has a distinct strip structure at the 1 / 2 thickness position, and the lath martensite structure of Example 1 is slightly refined compared to Comparative Example 1.

[0044] To investigate the effects of batch addition of rare earth elements on the toughness and plastic deformation capacity of cold-rolled duplex steel, 1.2 mm thick rare earth duplex steels Ce780 and 780DP (both in continuous annealing state) were taken and subjected to electron backscatter diffraction (EBSD) microstructure and crystal orientation detection analysis at the Analysis and Testing Center of Beijing University of Science and Technology.

[0045] When the orientation difference is greater than 15°, the orientation difference is greater than 15°, which is the large angle grain boundary (HAGB) of 780 grade dual-phase steel. The main purpose of the comparative detection and analysis of the proportion of large angle grain boundaries is: (1) Large angle grain boundaries microscopically hinder the path of low-temperature crack propagation, improve the bonding force, and thus improve the toughness of the material; (2) Large angle grain boundaries have a certain hindering effect on plastic deformation, and can also promote the occurrence of multi-system slip and twinning, thereby improving the plastic deformation capacity of the material. Therefore, the higher the proportion of large angle grain boundaries, the better the toughness and the better the plastic deformation capacity.

[0046] Electron backscattering diffraction (EBSD) analysis was performed on the rare-earth dual-phase steels obtained in Example 1 and Comparative Example 1: The rare earth dual-phase steels obtained in Example 1 and Comparative Example 1 are shown below. Figure 3 .

[0047] Figure 4 This is a diagram showing the test locations for electron backscatter diffraction (EBSD) analysis. As can be seen, the sample test area is 8 × 1.2 mm, and the subsurface and central layers were tested sequentially. A - 1 / 2 position of the central layer thickness; B - 1 / 4 position of the subsurface layer thickness. Magnification of the acquisition area: 2000x; Actual test area: 57.45 μm × 43.05 μm.

[0048] like Figure 5 The image shown is an EBSD layered image of Ce780-A1 (the first position in the central layer of Ce780 sample) from Example 1. When the central layer grain boundary asymmetry is 2-15 degrees, the boundary length is 341.8 μm, and the total boundary ratio is 49.7%; when the central layer grain boundary asymmetry is ≥15 degrees, the boundary length is 364.4 μm, and the total boundary ratio is 50.3%. When the subsurface grain boundary asymmetry is 2-15 degrees, the boundary length is 236.6 μm, and the total boundary ratio is 50.6%; when the subsurface grain boundary asymmetry is ≥15 degrees, the boundary length is 231.0 μm, and the total boundary ratio is 49.4%.

[0049] Figure 5In the diagram, the three colors represent: Red (Iron bcc): Body-centered cubic structure, corresponding to ferrite / martensite phase; Blue (Iron fcc): Face-centered cubic structure, corresponding to the austenitic phase; Gray (electronic image): Non-resolution area (usually a small inclusion, surface contamination, or severely deformed area); The image clearly shows that the dominant phase is the red bcc phase, which accounts for a very high proportion (>90%), and the material matrix is ​​composed of ferrite and martensite. The blue fcc phase is distributed discretely, uniformly dispersed in the bcc matrix as fine particles, and accounts for a very low proportion; there are no large areas of single-phase regions, and the overall microstructure is uniformly distributed. This morphology is beneficial for improving the material's strength, toughness, and formability.

[0050] Figure 5 The phase distribution is uniform throughout the entire field of view, with no obvious segregation or banded structure, indicating that the material has good compositional segregation and process control, which is beneficial to the consistency of performance.

[0051] like Figure 6 As shown in the contrast (BC) plot, the material exhibits a bimodal grain structure characterized by fine grains, clear grain boundaries, uniform structure, and no severe defects. This microstructure provides the material with high strength, good toughness, and stable processing properties.

[0052] Figure 7 The Euler angle color map (IPF / Euler color map) of EBSD is matched with the BC and phase distribution map.

[0053] Figure 8 The EBSD IPF-Z (inverse pole figure of the Z-axis) is used to characterize the crystal orientation distribution of grains relative to the sample normals (Z, Y, X directions). Combined with the comprehensive analysis of phase distribution, band contrast (BC), Euler angle color, and the IPF-Z orientation diagram, the material exhibits a multiphase microstructure with a body-centered cubic (BCC) matrix and a small amount of dispersed residual austenite. The grains show a typical bimodal distribution, are fine, without coarse recrystallized grains, have clear grain boundaries, and show sufficient recrystallization, resulting in good microstructure homogeneity. The IPF-Z orientation diagram shows highly random grain orientation with no obvious preferred texture and excellent isotropy. This microstructure effectively achieves a balance between strength and toughness: fine grains ensure matrix strength and improve toughness and formability; weak texture characteristics reduce processing anisotropy and significantly improve formability, providing reliable microstructure assurance for subsequent precision machining and service stability.

[0054] Figure 9 The EBSD phase map is a core characterization result for analyzing the microstructure of steel materials, and is related to... Figures 5-8 Use them together.

[0055] Figure 10 The diagram shows the proportion of large and small angle grain boundaries in EBSD. Green lines represent small-angle grain boundaries, with an orientation difference of 2°-15°, accounting for 49.7%; black lines represent large-angle grain boundaries, with an orientation difference of ≥15°, accounting for 50.3%. Large-angle grain boundaries can effectively change the crack propagation path, increase crack propagation resistance, and improve the toughness and impact resistance of the material. A high proportion of large-angle grain boundaries indicates excellent strength, toughness, and formability of the material.

[0056] Figure 11 The image shows the EBSD image of 780DP-A1 (the first position in the center layer of the 780DP sample) for Comparative Example 1. From left to right and top to bottom, the images are: EBSD layered image, contrast-enhanced (BC) image, Euler angle color map (IPF / Euler color map), IPF-Z (Z-axis inverse pole figure), phase map, and distribution map of large and small angle grain boundaries. Green lines represent small-angle grain boundaries with an orientation difference of 2°-15°, accounting for 52.3%; black lines represent large-angle grain boundaries with an orientation difference ≥15°, accounting for 47.7%. Large-angle grain boundaries can effectively change the crack propagation path, increase crack propagation resistance, and improve the toughness and impact resistance of the material. The product in this comparative example, with the addition of rare earth elements in a single step, has a low proportion of large-angle grain boundaries, resulting in relatively poorer strength, toughness, and formability compared to rare earth steel.

[0057] In Comparative Example 1, when the grain boundary asymmetry of the central layer is 2-15 degrees, the boundary length is 505.5 μm, and the total boundary ratio is 52.3%; when the grain boundary asymmetry is ≥15 degrees, the boundary length is 461.9 μm, and the total boundary ratio is 47.7%. When the grain boundary asymmetry of the subsurface layer is 2-15 degrees, the boundary length is 865.8 μm, and the total boundary ratio is 56.5%; when the grain boundary asymmetry of the subsurface layer is ≥15 degrees, the boundary length is 667.3 μm, and the total boundary ratio is 43.5%.

[0058] Depend on Figure 12 As can be seen, the results of the large-angle (≥15 degrees) grain boundary percentage test for Example 1 and Comparative Example 1 show that the large-angle grain boundary percentage of Example 1 is significantly higher than that of Comparative Example 1, indicating that Example 1 has better toughness and relatively better plastic deformation capability.

[0059] Electron backscattering diffraction (EBSD) crystal orientation detection analysis of cold-rolled rare-earth dual-phase steel Ce780 in Example 1 and 780DP in Comparative Example 1: As shown in Table 1, a total of 8 sets of data were analyzed, including: 2 sets at the 1 / 2 thickness position of Ce780, 2 sets at the 1 / 4 thickness position of Ce780; 2 sets at the 1 / 2 thickness position of 780DP, and 2 sets at the 1 / 4 thickness position of 780DP. The results show that the proportion of large-angle crystallization in Ce780 ranges from 49.4% to 56.4%, with an average of 52%, while the proportion of large-angle crystallization in 780DP ranges from 41.2% to 47.7%, with an average of 43.55%. In the samples provided in this study, the content of large-angle grain boundaries in Ce780 was 8.45% higher than that in 780DP of the same thickness. From the microstructure and crystal orientation results, the rare earth steel of Example 1 exhibits better toughness and plastic deformation capacity, resulting in improved forming performance for the corresponding client.

[0060] Table 1 Phase Proportion

[0061] Mechanical property testing and analysis: Ce780 of Example 1: tensile strength 810MPa, yield strength 470MPa, elongation 20.2%.

[0062] Comparative Example 1 780DP: tensile strength 750MPa, yield strength 421MPa, elongation 13%.

[0063] Comparative evaluation of the bending performance of Ce780 in Example 1 and 780DP in Comparative Example 1: To investigate the effect of rare earth elements on the forming properties of cold-rolled dual-phase steel, 1.2 mm thick rare earth dual-phase steels Ce780 and 780DP (both in continuous annealing state) were used, and the limiting bending angles under different bending radii were tested using a bending tester. No cracks were observed during longitudinal bending of 180 degrees at a bending radius of 0.2 mm (R=0.16t). The rare earth dual-phase steel of Example 1 showed improved transverse bending performance compared to 780DP of Comparative Example 1. Under bending radii of 0.2-0.8 mm (R=0.16t to R=0.67t), the limiting bending angle of rare earth Ce780 in Example 1 was 9-13° greater than that of 780DP in Comparative Example 1. See Tables 2 and 3 below for details.

[0064] Table 2 Longitudinal Bending

[0065] Table 3 Lateral Bending

[0066] As shown in Table 3, when bending laterally with a bending radius of 0.2 mm, the maximum bending angle is 147 degrees; when bending longitudinally with a bending radius of 0.2 mm, the maximum bending angle is 180 degrees, and there are no cracks. When bending laterally with a bending radius of 0.4mm, the maximum bending angle is 153 degrees; when bending longitudinally with a bending radius of 0.4mm, the maximum bending angle is 180 degrees, with no cracks. When bending laterally with a bending radius of 0.8mm, the maximum bending angle is 167 degrees without cracks; when bending longitudinally with a bending radius of 0.8mm, the maximum bending angle is 180 degrees without cracks. When bending laterally with a bending radius of 1.0 mm, the maximum bending angle is 180 degrees without cracking; when bending longitudinally with a bending radius of 1.0 mm, the maximum bending angle is 180 degrees without cracking.

[0067] The 1.2mm rare-earth dual-phase steel in Example 1 is used in components such as the lower crossbeam, front longitudinal beam, and sill reinforcement of passenger vehicle chassis. For example, Figure 13 As shown, the lower crossbeam of the vehicle chassis is formed by a 9-stage stamping process. The complex and demanding forming conditions are: the extreme sharp angle R = 1.7 mm, the stamping inner angle number = 56 degrees. Compared with the 780DP of Comparative Example 1, the straightness of the bend angle of Ce780 in Example 1 is improved by about 5%.

[0068] Example 2 A 780MPa grade rare earth high-strength dual-phase steel has the following chemical composition and mass percentage: C: 0.09%; Si: 0.20%; Mn: 1.8%; Als: 0.035%; Nb: 0.030%; Ti: 0.020%; Ce: 0.0028%; P: 0.010%; S: 0.002%; N: 0.0035%, with the remainder being Fe and unavoidable impurity elements.

[0069] A method for preparing 780MPa grade rare earth high-strength dual-phase steel includes the following steps: Step 1, converter smelting: 42t of scrap steel is added, the molten iron temperature is 1380℃, the final temperature is 1650℃, and the following raw materials are added: 2999.5kg of electrolytic manganese (grade DJMn99.9), 310kg of medium carbon ferromanganese (grade FeMn78C2.0), 3015kg of ferrosilicon manganese (grade FeMn64Si27), and 405kg of aluminum blocks (grade 1060, high purity aluminum, purity ≥99.97%). The converter smelting time is 85min, and the slag is blocked by a sliding plate when tapping the steel.

[0070] Step 2, LF refining: Heat to 1615℃, add 450kg of high-carbon ferromanganese (grade FeMn65C7.0), 116kg of ferroniobium (grade FeNb65), and 240kg of aluminum granules (grade 1060, high-purity aluminum, purity ≥99.97%), slag formation and desulfurization, LF refining time is 37min.

[0071] Step 3, RH refining: The molten steel exits the LF refining furnace and undergoes a three-stage RH refining process. First stage: Ultimate vacuum: 0.5 mbar, vacuum degassing, pure degassing time 9 min; Second stage: Add 185kg of high-carbon ferromanganese (grade FeMn65C7.0), 82kg of ferrotitanium (grade FeTi70), 18.5kg of aluminum granules (grade 1060, high-purity aluminum, purity ≥99.97%), 100kg of ferrosilicon (grade FeSi90), and 18.2kg of CeFe alloy (grade CeFe-90). The vacuum treatment time for this stage is 35min, and the ultimate vacuum degree is 1.4mbar. Third stage: Add 13kg of CeFe alloy (grade CeFe-90), vacuum treatment time for 7min, ultimate vacuum degree: 0.45mbar; molten steel exit temperature 1567℃.

[0072] Step 4, continuous casting: During the continuous casting process, the constant casting speed is 1.1m / min, the stopper rod rises slightly, and the liquid level fluctuation is controlled within ±5mm to obtain the billet.

[0073] Step 5, hot rolling: The billet is hot rolled with an entry temperature of 985℃, a final rolling temperature of 901℃, a coiling temperature of 551℃, a billet raw material thickness of 236mm, and a hot rolling thickness of 2.6mm.

[0074] Step 6, cold rolling: pickling reduction rate 54%, tension leveling rate 1.18%, rolling speed 337m / min, pickling speed 170m / min, finished product thickness 1.196mm.

[0075] Step 7, continuous annealing: thickness 1.196mm, width 1350mm, in the annealing furnace, the process speed is 95m / min, the slow cooling zone temperature is 649℃, the soaking zone temperature is 796℃, the rapid cooling zone temperature is 292℃, and the flattening elongation is 49.78%.

[0076] The rare-earth high-strength dual-phase steel obtained in this embodiment is applied to the automotive processing and manufacturing field. The automotive parts combine high strength and high formability, and are suitable for complex bending and stamping of automotive parts. Specifically, it is used in automotive parts such as the lower crossbeam, front longitudinal beam, and door sill reinforcement of passenger car chassis.

[0077] The rare earth high-strength dual-phase steel obtained in this embodiment has a tensile strength of 805 MPa, a yield strength of 461 MPa, and an elongation of 26.1%.

[0078] An automotive component is prepared from a 780MPa grade rare earth high-strength duplex steel according to this embodiment.

[0079] Example 3 A 780MPa grade rare earth high-strength dual-phase steel has the following chemical composition and mass percentage: C: 0.085%; Si: 0.16%; Mn: 2.05%; Als: 0.022%; Nb: 0.025%; Ti: 0.017%; Ce: 0.0026%; P: 0.011%; S: 0.003%; N: 0.003%, with the remainder being Fe and unavoidable impurity elements.

[0080] A method for preparing 780MPa grade rare earth high-strength dual-phase steel includes the following steps: Step 1, converter smelting: 38t of scrap steel is added, the molten iron temperature is 1280℃, the final temperature is 1635℃, and the following raw materials are added: 2985kg of electrolytic manganese (grade DJMn99.8), 300kg of medium carbon ferromanganese (grade FeMn75C2.0), 3008kg of ferrosilicon manganese (grade FeMn67Si23), and 395kg of aluminum blocks (grade 1050, high purity aluminum, purity ≥99.97%). The converter smelting time is 75min, and the slag is blocked by a sliding plate when tapping the steel.

[0081] Step 2, LF refining: Heat to 1610℃, add 440kg of high-carbon ferromanganese (grade FeMn70C7.0), 100kg of ferroniobium (grade FeNb70), and 230kg of aluminum granules (grade 1050, high-purity aluminum, purity ≥99.97%), slag formation and desulfurization, and the LF refining time is 32min.

[0082] Step 3, RH refining: The molten steel exits the LF refining furnace and undergoes a three-stage RH refining process. First stage: Ultimate vacuum: 1.0 mbar, vacuum degassing, pure degassing time 7 min; Second stage: Add 180kg of high-carbon ferromanganese (grade FeMn70C7.0), 75kg of ferrotitanium (grade FeTi70), 17kg of aluminum granules (grade 1050, high-purity aluminum, purity ≥99.97%), 105kg of ferrosilicon (grade FeSi75), and 15kg of CeFe alloy (grade CeFe-95). The vacuum treatment time for this stage is 25min, and the ultimate vacuum degree is 1.2mbar. Third stage: Add 10 kg of CeFe alloy (grade CeFe-95), vacuum treatment time for 6 min, ultimate vacuum degree: 0.35 mbar; molten steel exit temperature 1562℃.

[0083] Step 4, continuous casting: During the continuous casting process, the constant casting speed is 1.15m / min, the stopper rod rises slightly, and the liquid level fluctuation is controlled within ±5mm to obtain the billet.

[0084] Step 5, hot rolling: The billet is hot rolled with an inlet temperature of 970℃, a final rolling temperature of 900℃, a coiling temperature of 563℃, a billet raw material thickness of 236mm, and a hot rolling thickness of 4.0mm.

[0085] Step 6, cold rolling: pickling reduction rate 51%, tension leveling rate 0.82%, rolling speed 388m / min, pickling speed 114m / min, finished product thickness 2.003mm.

[0086] Step 7, continuous annealing: thickness 2.003mm, width 1325mm, in the annealing furnace, the process speed is 43m / min, the slow cooling zone temperature is 649℃, the soaking zone temperature is 787℃, the rapid cooling zone temperature is 261℃, and the flattening elongation is 49%.

[0087] The rare-earth high-strength dual-phase steel obtained in this embodiment is applied to the automotive processing and manufacturing field. The automotive parts combine high strength and high formability, and are suitable for complex bending and stamping of automotive parts. Specifically, it is used in automotive parts such as the lower crossbeam, front longitudinal beam, and door sill reinforcement of passenger car chassis.

[0088] The rare earth high-strength dual-phase steel obtained in this embodiment has a tensile strength of 808 MPa, a yield strength of 465 MPa, and an elongation of 22.5%.

[0089] An automotive component is prepared from a 780MPa grade rare earth high-strength duplex steel according to this embodiment.

[0090] Example 4 A 780MPa grade rare earth high-strength dual-phase steel has the following chemical composition and mass percentage: C: 0.08%; Si: 0.15%; Mn: 2.0%; Als: 0.020%; Nb: 0.030%; Ti: 0.015%; Ce: 0.0038%; P: 0.009%; S: 0.003%; N: 0.0031%, with the remainder being Fe and unavoidable impurity elements.

[0091] A method for preparing 780MPa grade rare earth high-strength dual-phase steel includes the following steps: Step 1, converter smelting: 30t of scrap steel is added, the molten iron temperature is 1240℃, the final temperature is 1620℃, and the following raw materials are added: 2950kg of electrolytic manganese (grade DJMn99.7), 280kg of medium carbon ferromanganese (grade FeMn78C2.0), 3000kg of ferrosilicon manganese (grade FeMn68Si22), and 370kg of aluminum blocks (grade 1A97, high purity aluminum, purity ≥99.97%). The converter smelting time is 60min, and the slag is blocked by a sliding plate when tapping the steel.

[0092] Step 2, LF refining: Heat to 1618℃, add 430kg of high-carbon ferromanganese (grade FeMn70C7.0), 113kg of ferroniobium (grade FeNb70), and 230kg of aluminum granules (grade 1A97, high-purity aluminum, purity ≥99.97%), slag formation and desulfurization, and the LF refining time is 30min.

[0093] Step 3, RH refining: The molten steel exits the LF refining furnace and undergoes a three-stage RH refining process. First stage: Ultimate vacuum: 0.1 mbar, vacuum degassing, pure degassing time 5 min; Second stage: Add 180kg of high-carbon ferromanganese (grade FeMn65C7.0), 70kg of ferrotitanium (grade FeTi70), 15kg of aluminum granules (grade 1A97, high-purity aluminum, purity ≥99.97%), 90kg of ferrosilicon (grade FeSi65), and 19kg of CeFe alloy (grade CeFe-90). The vacuum treatment time for this stage is 20min, and the ultimate vacuum degree is 1.1mbar. Third stage: Add 14 kg of CeFe alloy (grade CeFe-90), vacuum treatment time for 5 min, ultimate vacuum degree: 0.3 mbar; molten steel exit temperature 1560℃.

[0094] Step 4, continuous casting: During the continuous casting process, the constant casting speed is 1.2m / min, the stopper rod rises slightly, and the liquid level fluctuation is controlled within ±5mm to obtain the billet.

[0095] Step 5, hot rolling: The billet is hot rolled with an inlet temperature of 950℃, a final rolling temperature of 890℃, a coiling temperature of 545℃, a billet raw material thickness of 236mm, and a hot rolling thickness of 4.1mm.

[0096] Step 6, cold rolling: pickling reduction rate 51%, tension leveling rate 0.37%, rolling speed 256m / min, pickling speed 178m / min, finished product thickness 2.009mm.

[0097] Step 7, continuous annealing: thickness 2.009mm, width 1393mm, in the annealing furnace, the process speed is 50m / min, the slow cooling zone temperature is 643℃, the soaking zone temperature is 810℃, the rapid cooling zone temperature is 261℃, and the flattening elongation is 49.82%.

[0098] The rare-earth high-strength dual-phase steel obtained in this embodiment is applied to the automotive processing and manufacturing field. The automotive parts combine high strength and high formability, and are suitable for complex bending and stamping of automotive parts. Specifically, it is used in automotive parts such as the lower crossbeam, front longitudinal beam, and door sill reinforcement of passenger car chassis.

[0099] The rare earth high-strength dual-phase steel obtained in this embodiment has a tensile strength of 817 MPa, a yield strength of 480 MPa, and an elongation of 25.5%.

[0100] An automotive component is prepared from a 780MPa grade rare earth high-strength duplex steel according to this embodiment.

[0101] Example 5 A 780MPa grade rare earth high-strength dual-phase steel has the following chemical composition and mass percentage: C: 0.010%; Si: 0.25%; Mn: 2.3%; Als: 0.055%; Nb: 0.035%; Ti: 0.025%; Ce: 0.0029%; P: 0.011%; S: 0.001%; ​​N: 0.0028%, with the remainder being Fe and unavoidable impurity elements.

[0102] A method for preparing 780MPa grade rare earth high-strength dual-phase steel includes the following steps: Step 1, converter smelting: 50t of scrap steel is added, the molten iron temperature is 1480℃, the final temperature is 1660℃, and the following raw materials are added: 3000kg of electrolytic manganese (grade DJMn99.7), 330kg of medium carbon ferromanganese (grade FeMn78C2.0), 3050kg of ferrosilicon manganese (grade FeMn68Si22), and 420kg of aluminum blocks (grade 1060, high purity aluminum, purity ≥99.97%). The converter smelting time is 100min, and the steel is tapped using a sliding plate to block slag.

[0103] Step 2, LF refining: Heat to 1630℃, add 480kg of high-carbon ferromanganese (grade FeMn65C7.0), 120kg of ferroniobium (grade FeNb65), and 250kg of aluminum granules (grade 1060, high-purity aluminum, purity ≥99.97%), slag formation and desulfurization, LF refining time is 40min.

[0104] Step 3, RH refining: The molten steel exits the LF refining furnace and undergoes a three-stage RH refining process. First stage: Ultimate vacuum: 1.2 mbar, vacuum degassing, pure degassing time 12 min; Second stage: Add 190kg of high-carbon ferromanganese (grade FeMn65C7.0), 85kg of ferrotitanium (grade FeTi70), 20kg of aluminum granules (grade 1060, high-purity aluminum, purity ≥99.97%), 110kg of ferrosilicon (grade FeSi65), and 20kg of CeFe alloy (grade CeFe-90). The vacuum treatment time for this stage is 40min, and the ultimate vacuum degree is 1.5mbar. Third stage: Add 15kg of CeFe alloy (grade CeFe-90), vacuum treatment time for 10min, ultimate vacuum degree: 0.5mbar; molten steel exit temperature 1570℃.

[0105] Step 4, continuous casting: During the continuous casting process, the constant casting speed is 1.15m / min, the stopper rod rises slightly, and the liquid level fluctuation is controlled within ±5mm to obtain the billet.

[0106] Step 5, hot rolling: The billet is hot rolled with an inlet temperature of 1000℃, a final rolling temperature of 910℃, a coiling temperature of 550℃, a billet raw material thickness of 236mm, and a hot rolling thickness of 3.0mm.

[0107] Step 6, cold rolling: pickling reduction rate 55%, tension leveling rate 1.10%, rolling speed 412m / min, pickling speed 191m / min, finished product thickness 1.35mm.

[0108] Step 7, continuous annealing: thickness 1.35mm, width 1450mm, in the annealing furnace, the process speed is 110m / min, the slow cooling zone temperature is 651℃, the soaking zone temperature is 795℃, the rapid cooling zone temperature is 305℃, and the flattening elongation is 51%.

[0109] The rare-earth high-strength dual-phase steel obtained in this embodiment is applied in the automotive processing and manufacturing field. The automotive parts combine high strength and high formability, and are suitable for complex bending and stamping parts of automobiles. It is used in automotive parts such as the lower crossbeam, front longitudinal beam, and door sill reinforcement of passenger car chassis.

[0110] The rare earth high-strength dual-phase steel obtained in this embodiment has a tensile strength of 793 MPa, a yield strength of 445 MPa, and an elongation of 25.7%.

[0111] An automotive component is prepared from a 780MPa grade rare earth high-strength duplex steel according to this embodiment.

[0112] Comparative Example 2 The only difference between this comparative example and Example 2 is that: In the RH refining process, the CeFe alloy is fed in a single batch, specifically: First stage: Ultimate vacuum: 0.5 mbar, vacuum degassing, pure degassing time 9 min; Second stage: Add 185kg of high-carbon ferromanganese (grade FeMn65C7.0), 82kg of ferrotitanium (grade FeTi70), 18.5kg of aluminum granules (grade 1060, high-purity aluminum, purity ≥99.97%), 100kg of ferrosilicon (grade FeSi90), and 31.2kg of CeFe alloy (grade CeFe-90). The vacuum treatment time for this stage is 35min, and the ultimate vacuum degree is 1.4mbar. Third stage: Vacuum treatment time in this stage is 7 minutes, ultimate vacuum degree: 0.45 mbar; molten steel exit temperature is 1567℃.

[0113] The rare earth high-strength dual-phase steel in this comparative example has a tensile strength of 740 MPa, a yield strength of 425 MPa, and an elongation of 13.5%.

[0114] Comparative Example 3 The only difference between this comparative example and Example 4 is that: In the RH refining process, the CeFe alloy is fed in a single batch, specifically: First stage: Ultimate vacuum: 0.1 mbar, vacuum degassing, pure degassing time 5 min; Second stage: Add 180kg of high-carbon ferromanganese (grade FeMn65C7.0), 70kg of ferrotitanium (grade FeTi70), 15kg of aluminum granules (grade 1A97, high-purity aluminum, purity ≥99.97%), 90kg of ferrosilicon (grade FeSi65), and 33kg of CeFe alloy (grade CeFe-90). The vacuum treatment time for this stage is 20min, and the ultimate vacuum degree is 1.1mbar. Third stage: Vacuum treatment time in this stage is 5 minutes, ultimate vacuum degree: 0.3 mbar; molten steel exit temperature is 1560℃.

[0115] The rare earth high-strength dual-phase steel obtained in this comparative example has a tensile strength of 782 MPa, a yield strength of 436 MPa, and an elongation of 12.0%.

[0116] Comparative Example 4 The only difference between this comparative example and Example 5 is that: In the RH refining process, the CeFe alloy is fed in a single batch, specifically: First stage: Ultimate vacuum: 1.2 mbar, vacuum degassing, pure degassing time 12 min; Second stage: Add 190kg of high-carbon ferromanganese (grade FeMn65C7.0), 85kg of ferrotitanium (grade FeTi70), 20kg of aluminum granules (grade 1060, high-purity aluminum, purity ≥99.97%), 110kg of ferrosilicon (grade FeSi65), and 35kg of CeFe alloy (grade CeFe-90). The vacuum treatment time for this stage is 40min, and the ultimate vacuum degree is 1.5mbar. Third stage: Vacuum treatment time in this stage is 10 minutes, ultimate vacuum degree: 0.5 mbar; molten steel exit temperature is 1570℃.

[0117] The rare earth high-strength dual-phase steel obtained in this comparative example has a tensile strength of 728 MPa, a yield strength of 380 MPa, and an elongation of 15.0%.

[0118] Comparative Example 5 The only difference between this comparative example and Example 1 is that the first stage of RH refining in step three is omitted.

[0119] The rare earth high-strength dual-phase steel obtained in this comparative example has a tensile strength of 735 MPa, a yield strength of 407 MPa, and an elongation of 14.0%.

[0120] In the foregoing description of exemplary embodiments of the invention, various features of the invention are sometimes grouped together in a single embodiment, figure, or description thereof. However, this disclosure should not be construed as reflecting an intention that the claimed invention requires more features than expressly recited in each claim. Rather, as reflected in the claims, the inventive aspect lies in fewer than all features of the foregoingly disclosed embodiments. Therefore, the claims, following the detailed description, are hereby expressly incorporated into that detailed description, wherein each claim itself is a separate embodiment of the invention.

[0121] Although the invention has been described with reference to a limited number of embodiments, those skilled in the art will understand from the foregoing description that other embodiments are conceivable within the scope of the invention described herein. Furthermore, it should be noted that the language used in this specification has been chosen primarily for readability and instructional purposes, and not for the purpose of interpreting or limiting the subject matter of the invention. Therefore, many modifications and variations will be apparent to those skilled in the art without departing from the scope and spirit of the appended claims. The disclosure of the invention is illustrative and not restrictive, and the scope of the invention is defined by the appended claims.

[0122] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A 780MPa grade rare earth high-strength dual-phase steel, characterized in that, The chemical composition and its mass percentage are as follows: C: 0.08-0.10%; Si: 0.15-0.25%; Mn: 1.8-2.4%; Als: 0.020-0.055%; Nb: 0.025-0.035%; Ti: 0.015-0.025%; Ce: 0.0026-0.0045%, P: ≤0.012%; S: ≤0.004%; N: ≤0.004%, the remainder being Fe and unavoidable impurity elements.

2. The 780MPa grade rare earth high-strength dual-phase steel according to claim 1, characterized in that, Tensile strength: 793-817MPa, fluctuation range: 24MPa; Yield strength: 445-480MPa, fluctuation range: 35MPa; Elongation: 20.2%-26.1%.

3. The 780MPa grade rare earth high-strength dual-phase steel according to claim 1, characterized in that, The proportion of large-angle grain boundaries is 49.4%-56.4%, with large angles being ≥15°.

4. The 780MPa grade rare earth high-strength dual-phase steel according to claim 1, characterized in that, When bending laterally with a bending radius of 0.2mm, the maximum bending angle is 147 degrees; when bending longitudinally with a bending radius of 0.2mm, the maximum bending angle is 180 degrees, with no cracks. When bending laterally, with a bending radius of 0.4mm, the maximum bending angle is 153 degrees; when bending longitudinally, with a bending radius of 0.4mm, the maximum bending angle is 180 degrees, with no cracks. When bending laterally with a bending radius of 0.8mm, the maximum bending angle is 167 degrees without cracks; when bending longitudinally with a bending radius of 0.8mm, the maximum bending angle is 180 degrees without cracks. When bending laterally with a bending radius of 1.0 mm, the maximum bending angle is 180 degrees without cracking; when bending longitudinally with a bending radius of 1.0 mm, the maximum bending angle is 180 degrees without cracking.

5. A method for preparing a 780MPa grade rare earth high-strength dual-phase steel according to any one of claims 1-4, characterized in that, Includes the following steps: Step 1, converter smelting: 30-50t of scrap steel is added, the molten iron temperature is 1240-1480℃, the final temperature is 1620-1660℃, and the following raw materials are added: 2950-3000kg of electrolytic manganese, 280-330kg of medium carbon ferromanganese, 3000-3050kg of ferrosilicon manganese, and 370-420kg of aluminum blocks. The converter smelting time is 60-100min. Step 2, LF refining: Heat to 1610-1630℃, add 430-480kg of high-carbon ferromanganese, 100-120kg of ferroniobium, and 230-250kg of aluminum granules. The LF refining time is 30-40 minutes. Step 3, RH refining: The molten steel exits the LF refining furnace and undergoes a three-stage RH refining process. First stage: Ultimate vacuum: 0-1.2 mbar, vacuum degassing, pure degassing time 5-12 min; Second stage: Add 180-190kg of high-carbon ferromanganese, 70-85kg of ferrotitanium, 15-20kg of aluminum granules, 90-110kg of ferrosilicon, and 15-20kg of CeFe alloy. The vacuum treatment time for this stage is 20-40 minutes, and the ultimate vacuum degree is 1.1-1.5mbar. Third stage: Add 10-15 kg of CeFe alloy. Vacuum treatment time for this stage is 5-10 min. Ultimate vacuum degree: 0.3-0.5 mbar; molten steel exit temperature 1560-1570℃. Step 4, continuous casting: During the continuous casting process, a constant casting speed of 1.1-1.2 m / min is maintained to obtain the cast billet; Step 5, hot rolling: The billet is hot rolled with an entry temperature of 950-1000℃, a final rolling temperature of 890-910℃, a coiling temperature of 545-563℃, and a hot-rolled thickness of 2.5-4.1mm. Step 6, cold rolling: pickling reduction rate 51%-55%, tension leveling rate 0.37-1.18%, rolling speed 256-412m / min, pickling speed 114-191m / min, finished product thickness 1.2-2.0mm; Step 7: Revert.

6. The preparation method according to claim 5, characterized in that, In step one, slag is blocked by a sliding plate during steel tapping.

7. The preparation method according to claim 5, characterized in that, In step seven, in the annealing furnace, the process speed is 43-110 m / min, the temperature of the slow cooling section is 643-651℃, the temperature of the soaking section is 787-810℃, the temperature of the rapid cooling section is 261-305℃, and the flattening elongation is 49-51%.

8. The application of a 780MPa grade rare earth high-strength duplex steel according to any one of claims 1-3 in automotive parts.

9. The application according to claim 8, characterized in that, Automotive parts combine high strength and high formability, and are suitable for complex bending and stamping of automotive components; complex bending and stamping of automotive components include passenger car chassis lower crossbeams, front longitudinal beams, and door sill reinforcements.

10. A component for automobiles, characterized in that, The parts are prepared from a 780MPa grade rare earth high-strength dual-phase steel as described in any one of claims 1-4, and are stamped for automotive parts. The forming conditions are as follows: when the forming direction is transverse: the limit angle R ≥ 0.2mm, and the number of stamping internal angles ≤ 147 degrees; when the limit angle R ≥ 1.0mm, and the number of stamping internal angles meets the requirement of forming at any angle from 0 to 180 degrees; when the forming direction is longitudinal: the limit angle R ≥ 0.2mm, and the number of stamping internal angles meets the requirement of forming at any angle from 0 to 180 degrees.

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