High-strength and high-elongation cold-rolled isotropic steel and manufacturing method thereof

Abstract

The invention relates to the technical field of metal materials, in particular to high-strength and high-elongation cold-rolled isotropic steel and a manufacturing method thereof. The alloy comprises the following chemical components in percentage by weight: 0.004%-0.008% of C, less than or equal to 0.01% of Si, less than or equal to 0.1% of Mn, 0.02%-0.08% of Als, 0.1%-0.2% of Cu, 0.02%-0.04% of Nb, Plt and the balance of Fe. 0.012%, Slt; 0.008%, N < = 0.003%, and the balance Fe. Through the specific component design and the optimized hot rolling-cold rolling-annealing process, the yield strength of the steel is larger than or equal to 110 MPa, the tensile strength is larger than or equal to 300 MPa, the ductility is larger than or equal to 48%, and delta r is smaller than or equal to 0.15. Compared with common IF steel, the IF steel has higher strength and ductility and better deep drawability.

Description

Technical Field

[0001] This invention relates to the field of metallic materials technology, specifically to a high-strength, high-elongation cold-rolled isotropic steel and its manufacturing method. Background Technology

[0002] Since the beginning of the 21st century, with the rapid development of the global automotive industry, especially the increasingly stringent regulations on lightweighting, safety, and environmental protection, automotive steel has faced unprecedented performance challenges. Traditional automotive body panels not only need to possess high strength to meet crash safety requirements, but also must have excellent stamping formability to adapt to the processing needs of complex-shaped parts. This demand has driven the rapid development of automotive steel from early mild steel (such as DC04) to high-strength IF steel (HSS-IF), advanced high-strength steel (AHSS), and even ultra-high-strength steel (UHSS). However, while pursuing higher strength levels, materials must maintain good total elongation (TE) and plastic strain ratio (r-value) to ensure the feasibility of key processes such as deep drawing and hole expansion.

[0003] Although traditional IF steel has excellent deep-drawing properties (such as r... 90 While its strength is typically ≤2.0 MPa, its low strength (usually ≤300 MPa) and defects such as earing and orange peeling under high strain conditions make it difficult to meet the performance requirements of modern car body structural components. In particular, the yield strength (YS) of ordinary IF steel (such as DC06) is usually below 180 MPa, and its tensile strength (TS) is ≤300 MPa. At ultra-high strength levels (≥500 MPa), the elongation decreases significantly, resulting in a reduced forming limit. At the same time, softening zones are prone to occur during welding, and the performance of phosphating and electrophoretic coating is also affected.

[0004] To overcome these technological bottlenecks, steel companies and research institutions have been focusing on developing new types of cold-rolled steel sheets that combine high strength, high elongation, and excellent deep-drawing performance in recent years. Summary of the Invention

[0005] To overcome the shortcomings of the prior art, the present invention provides a high-strength, high-elongation cold-rolled isotropic steel and its manufacturing method, which, compared with ordinary IF steel, has higher strength and elongation while having better deep-drawing performance.

[0006] To achieve the above objectives, the present invention employs the following technical solution: A high-strength, high-elongation cold-rolled isotropic steel is composed of the following chemical composition in weight percentages: C: 0.004%~0.008%, Si≤0.01%, Mn≤0.1%, Als: 0.02%~0.08%, Cu: 0.1%~0.2%, Nb: 0.02%~0.04%, P<0.012%, S<0.008%, N≤0.003%, balance Fe.

[0007] The effect of selecting the above alloying elements and their contents: 1. Carbon (C) is a common element in steel. It can expand the austenite phase region and improve the strength of steel, but it significantly reduces the plasticity of steel. This invention adopts an ultra-low carbon design to improve the plasticity of steel. Considering the smelting cost, the C content is precisely controlled. Therefore, this invention precisely controls the C content to 0.004%~0.008%.

[0008] 2. Nitrogen (N), a common gaseous element in steel, can form aluminum nitride to refine grains, but it also increases the steel's aging sensitivity. Therefore, this invention precisely controls the N content to ≤0.003%.

[0009] 3. Si is also a common element in steel, increasing its strength but significantly reducing its surface quality. Ultra-deep drawing parts generally have high requirements for steel surface quality. Based on cost factors and the need for high-quality steel surface, this invention precisely controls the Si content to ≤0.01%.

[0010] 4. P and S elements have no benefit to the mechanical properties of steel plates, especially elongation. P should be controlled to <0.01% and S to <0.008%.

[0011] 5. Al has a solid solution strengthening effect, and when it combines with nitrogen to form AlN, it can refine grains and provide precipitation strengthening, but it increases the inclusion content of the steel and reduces its plasticity. Therefore, this invention precisely controls the Al content to be between 0.02% and 0.08%.

[0012] 6. Manganese (Mn) is a common element in steel, which can refine grains and improve strength. However, manganese significantly increases the anisotropy of steel and reduces its deep-drawing performance. Therefore, a low-manganese design is adopted. Thus, this invention precisely controls the Mn content to ≤0.1%.

[0013] 7. Cu expands the austenite phase region, providing solid solution strengthening and precipitation strengthening effects, thus improving the weather resistance of steel. However, high Cu content can lead to hot brittleness. Therefore, this invention precisely controls the Cu content to 0.1%~0.2%.

[0014] 8. As a microalloying element, Nb not only refines grains and improves strength, but also significantly reduces the anisotropy of steel. Therefore, this invention precisely controls the Nb content to be between 0.02% and 0.04%.

[0015] The aforementioned high-strength, high-elongation cold-rolled isotropic steel has a yield strength ≥110MPa, tensile strength ≥300MPa, and elongation ≥48%. The steel's plastic strain ratio anisotropy is |Δr|≤0.15, Δr=(r0+r 90 -2r 45 ) / 2, where r0, r 90 r 45 These are the plastic strain ratios in the rolling direction, the 45° direction, and the transverse direction, respectively.

[0016] The manufacturing method for the aforementioned high-strength, high-elongation cold-rolled isotropic steel specifically includes the following steps: 1. Hot rolling process: The intermediate billet is heated to 1150~1250℃ and homogenized for 30~40min. After homogenization, the intermediate billet is rolled in four passes to a thickness of 5.5mm, with a final rolling temperature of 860~900℃. After rolling, it is cooled to 590~610℃ at a cooling rate of 15~25℃ / s. After holding at 550~570℃ for 15min, it is cooled in the furnace to simulate the coiling process.

[0017] The intermediate billet consists of the following chemical composition by weight percentage: C: 0.004%~0.008%, Si≤0.01%, Mn≤0.1%, Als: 0.02%~0.08%, Cu: 0.1%~0.2%, Nb: 0.02%~0.04%, P<0.012%, S<0.008%, N≤0.003%, balance Fe.

[0018] 2) Pickling and cold rolling processes: The hot-rolled plate is pickled and then cold-rolled, with a cold rolling reduction rate of ≥70%; the thickness of the cold-rolled steel plate is less than 1.65mm.

[0019] 3) Continuous annealing process: The following segmented temperature control process is adopted: Heating stage: Heat to 790~830℃ at a rate of 9~11℃ / s, and hold for 150s; Slow cooling stage: Cool to 670~690℃ at a rate of 14~16℃ / s; Rapid cooling stage: Cooling to the aging temperature of 345~355℃ at a rate of 28~32℃ / s; Aging treatment: Hold at 345~355℃ for 540~560s; Final cooling stage: Cool to ≤200℃ at a rate of ≥20℃ / s.

[0020] The continuous annealing process is carried out in a protective atmosphere, which is a nitrogen-hydrogen mixture, in which the hydrogen gas component is 5-15%.

[0021] Compared with the prior art, the beneficial effects of the present invention are: 1. This invention, through an innovative ultra-low carbon design (C: 0.004%~0.008%) combined with Nb microalloying (0.02%~0.04%) and Cu alloying (0.1%~0.2%), significantly improves material strength while ensuring high plasticity. The resulting steel plate exhibits a yield strength of 135-200 MPa, a tensile strength of 300-350 MPa, and an elongation after fracture of 42.5%-52.5%, with the optimal embodiment achieving an elongation of 48.5%-52.5%, thus achieving the best balance between strength and plasticity.

[0022] 2. This invention employs a low-manganese design (Mn≤0.1%) and an optimized hot-rolling-cold-rolling-annealing process, resulting in materials exhibiting excellent isotropic characteristics. The average plastic strain ratio (raverage) is stable within the range of 1.40-1.60, and the anisotropy ratio (Δr / raverage) is controlled between 2% and 8%, which translates to an absolute value |Δr|≤0.15. Compared with conventional IF steel, the deep-drawing uniformity is improved by more than 30%, effectively solving the technical problem of significant anisotropy in traditional high-strength steel.

[0023] 3. The present invention adopts an ultra-low silicon design (Si≤0.01%) to fundamentally avoid surface oxide scale defects. Strict sulfur and phosphorus control (S<0.008%, P<0.012%) significantly improves the surface smoothness of cold-rolled sheet. The cooling process with a cooling rate of 28~32℃ / s inhibits surface oxidation, and the Ra value can be controlled below 0.5μm.

[0024] 4. This invention achieves a uniform and fine grain structure through a precise four-pass rolling process (final rolling temperature 880±20℃) combined with rapid cooling at 20±5℃ / s. The innovative multi-stage continuous annealing process (including holding at 810±20℃, segmented cooling, and aging treatment at 350±5℃) ensures complete recrystallization and a uniform structure, giving the material excellent formability, making it particularly suitable for stamping complex-shaped parts.

[0025] 5. This invention uses a large reduction rate of ≥70% for cold rolling, which significantly improves the dislocation density and provides a driving force for subsequent recrystallization.

[0026] In summary, this invention achieves breakthroughs in material performance through innovative composition design and optimized manufacturing processes, and more importantly, through deep synergy between composition and manufacturing processes and a multi-scale control mechanism. A cold-rolled isotropic steel with high strength, high elongation, and low anisotropy has been successfully developed, ensuring a yield strength ≥110MPa, tensile strength ≥300MPa, elongation of 48%, and |Δr| ≤0.15. Compared to ordinary IF steel, it possesses higher strength and elongation while exhibiting better deep-drawing performance. Its overall performance is significantly superior to ordinary IF steel, providing an ideal material solution for complex formed parts in fields such as automotive lightweighting and high-end home appliances. Detailed Implementation

[0027] This invention discloses a high-strength, high-elongation cold-rolled isotropic steel and its manufacturing method. Those skilled in the art can refer to the content of this document and appropriately modify the process parameters to achieve the desired result. It should be particularly noted that all similar substitutions and modifications are obvious to those skilled in the art and are considered to be included in this invention. The methods and applications of this invention have been described through preferred embodiments, and those skilled in the art can clearly modify or appropriately change and combine the methods and applications described herein without departing from the content, spirit, and scope of this invention to realize and apply the technology of this invention.

[0028] To improve strength and significantly reduce the anisotropy of the r-value, thus meeting the requirements for strength and deep drawing performance, this invention utilizes a balanced composition of C, Mn, Cu, and Nb, along with key production technologies for high-strength, high-elongation cold-rolled isotropic steel. The inventors conducted extensive and systematic experimental research in several aspects, including alloy element screening and proportioning, steel cleanliness control, and optimization and parameter selection of efficient rolling processes. Ultimately, they determined the alloy element proportions and production processes that meet the objectives of this invention. The chemical composition of the steel plate in this embodiment is shown in Table 1, the hot rolling process parameters and cold rolling reduction rate are shown in Table 2, the continuous annealing process is shown in Table 3, and the steel plate properties are shown in Table 4.

[0029] Table 1 Chemical composition (wt%) of steel plates in embodiments of the present invention Table 2 Hot rolling process parameters and cold rolling reduction rates of embodiments of the present invention Table 3 Continuous annealing process parameters of the embodiments of the present invention Table 4. Steel plate properties of embodiments of the present invention The high-strength, high-elongation cold-rolled isotropic steel provided by this invention achieves a breakthrough improvement in mechanical properties through innovative composition design and optimized hot-rolling-cold-rolling-annealing process. As shown in Table 4, this steel exhibits excellent comprehensive performance: yield strength (Rp0.2) in the range of 120~200MPa, tensile strength (Rm) reaching 300~350MPa, and elongation after fracture (A50) as high as 42.5%~52.5%, with the optimal embodiment reaching 48.5%~52.5%. Particularly noteworthy is that while maintaining high plasticity, the material exhibits excellent deep-drawing performance, with the average plastic strain ratio (r_average) stable in the range of 1.40~1.60, and the anisotropy ratio (Δr / r_average) controlled between 2%~8%, which translates to an absolute value |Δr|≤0.15, demonstrating excellent isotropic characteristics. Compared to conventional IF steel, the steel grade of this invention maintains a significant advantage in elongation (by approximately 8-12 percentage points) while increasing strength by over 20%, and simultaneously improves deep-drawing uniformity by over 30%, completely overcoming the technical challenges of insufficient plasticity and significant anisotropy in traditional high-strength steel. This unique combination of properties makes this material particularly suitable for applications with stringent requirements for formability and strength, such as automotive structural components and precision appliance housings, providing a new material solution for lightweight design. Example data shows that the performance indicators of this steel grade are stable and reliable, with good batch repeatability, fully meeting the requirements of industrial production.

[0030] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in the present invention, based on the technical solution and inventive concept of the present invention, should be covered within the scope of protection of the present invention.

Claims

1. A high-strength, high-elongation cold-rolled isotropic steel, characterized in that, It is composed of the following chemical components in weight percentage: C: 0.004%~0.008%, Si≤0.01%, Mn≤0.1%, Als: 0.02%~0.08%, Cu: 0.1%~0.2%, Nb: 0.02%~0.04%, P<0.012%, S<0.008%, N≤0.003%, balance Fe.

2. The high-strength, high-elongation cold-rolled isotropic steel according to claim 1, characterized in that, The steel has a yield strength ≥110MPa and a tensile strength ≥300MPa.

3. The high-strength, high-elongation cold-rolled isotropic steel according to claim 1, characterized in that, The elongation of the steel is ≥48%.

4. The high-strength, high-elongation cold-rolled isotropic steel according to claim 1, characterized in that, The anisotropy of the plastic strain ratio of steel is |Δr|≤0.15, Δr=(r0+r 90 -2r 45 ) / 2, where r0, r 90 r 45 These are the plastic strain ratios in the rolling direction, the 45° direction, and the transverse direction, respectively.

5. A method for manufacturing high-strength, high-elongation cold-rolled isotropic steel as described in any one of claims 1-4, characterized in that, The manufacturing method specifically includes the following steps: 1) Hot rolling process: The intermediate billet is heated to 1150~1250℃ and homogenized for 30~40 minutes, and the final rolling temperature is 860~900℃. After rolling, the temperature is cooled to 590-610℃ at a cooling rate of 15-25℃ / s; After holding at 550~570℃ for 15 minutes, the furnace is cooled to simulate the winding process. 2) Pickling and cold rolling processes: Hot-rolled plates are pickled and then cold-rolled with a cold rolling reduction rate of ≥70%. 3) Continuous annealing process: The following segmented temperature control process is adopted: Heating stage: Heat to 790~830℃ at a rate of 9~11℃ / s, and hold for 150s; Slow cooling stage: Cool to 670~690℃ at a rate of 14~16℃ / s; Rapid cooling stage: Cooling to the aging temperature of 345~355℃ at a rate of 28~32℃ / s; Aging treatment: Hold at 345~355℃ for 540~560s; Final cooling stage: Cool to ≤200℃ at a rate of ≥20℃ / s.

6. The method for manufacturing high-strength, high-elongation cold-rolled isotropic steel according to claim 5, characterized in that, 1) After homogenization, the intermediate billet is rolled to a thickness of 5.5 mm in four passes.

7. The method for manufacturing high-strength, high-elongation cold-rolled isotropic steel according to claim 5, characterized in that, 1) In the hot rolling process, the intermediate billet is composed of the following chemical composition by weight percentage: C: 0.004%~0.008%, Si≤0.01%, Mn≤0.1%, Als: 0.02%~0.08%, Cu: 0.1%~0.2%, Nb: 0.02%~0.04%, P<0.012%, S<0.008%, N≤0.003%, balance Fe.

8. The method for manufacturing high-strength, high-elongation cold-rolled isotropic steel according to claim 5, characterized in that, 2) In this case, the thickness of the cold-rolled steel plate is less than 1.65mm.

9. The method for manufacturing high-strength, high-elongation cold-rolled isotropic steel according to claim 5, characterized in that, In step 3), the continuous annealing process is carried out in a protective atmosphere, and the protective gas is a nitrogen-hydrogen mixture, in which the hydrogen gas component is 5% to 15%.

Images