Corrosion-resistant 700mpa-grade light-weight ferritic steel and method of making same

Abstract

The present application relates to the technical field of cold-rolled automobile steel production, and particularly relates to a corrosion-resistant 700MPa-grade light ferrite steel and a preparation method thereof, wherein the chemical composition of the steel is C: 0.045%-0.055%, Mn: 0.40%-0.55%, Si: 0.40%-0.55%, Al: 5.0%-6.0%, P≤0.01%, S≤0.01%, N≤0.005%, Nb: 0.2%-0.3%, Ti: 0.2%-0.3%, V: 0.2%-0.3%, Cr: 0.1%-0.5%, and the balance is Fe and impurities. The light element Al and the corrosion-resistant element Cr are added on the basis of the traditional cold-rolled ferrite steel, the alloy cost is low, and the requirements of automobile lightening and comprehensive performance are met.

Description

Technical Field

[0001] This invention relates to the field of cold-rolled automotive steel production technology, and in particular to a corrosion-resistant 700MPa grade lightweight ferritic steel and its preparation method. Background Technology

[0002] In recent years, with increasingly stringent energy conservation and environmental protection requirements, the automotive industry is moving towards lightweighting and electrification. The research and widespread application of high-strength and ultra-high-strength automotive steels are effective ways to achieve vehicle lightweighting. However, with the continuous development of automotive steel technology, it is becoming increasingly difficult to reduce the weight of automotive parts simply by increasing the strength of steel sheets. Although people have also tried to use lightweight materials such as aluminum, magnesium, and carbon fiber to manufacture automotive parts and complete vehicles, their application has been limited due to problems such as high cost, insufficient processing performance, and poor weldability.

[0003] The present invention aims to develop a corrosion-resistant 700MPa grade lightweight ferritic steel that can be produced on a traditional cold rolling production line at extremely low alloy cost. It can meet both the requirements of low cost and the requirements of high lightweight and corrosion resistance, providing a feasible technical solution for automobile manufacturers and steel companies.

[0004] Chinese patent application CN118703904A discloses a "low-cost 700MPa grade quenched and tempered high-strength steel and its production method." The main chemical composition of the steel is: C: 0.13-0.15%, Si: 0.20-0.30%, Mn: 1.4-1.6%, P≤0.012%, S≤0.002%, Ti: 0.02-0.03%, Cr: 0.25-0.35%, Nb: 0.02-0.03%, B: 0.0008-0.0015%, N≤0.005%, with the remainder being Fe and unavoidable impurities. While the product achieves a strength exceeding 700MPa, it does not address the issue of lightweight vehicle bodies, making it difficult to meet the requirements for lightweight automotive manufacturing.

[0005] Chinese patent application CN115323278A discloses "a low-density steel with a yield strength of 700MPa and its heat treatment method." The main chemical composition of the steel is: C: 0.85-1.15%, Mn: 25-35%, Al: 9.0-11.0wt%, Nb: 0-0.2%, Mo: 0-0.2%, V: 0-0.2%, Ti: 0-0.2%, and Zr: 0-0.2%, wherein the total amount of Nb, Mo, V, Ti, and Zr is not less than 0.05%, and the balance is Fe and unavoidable impurities. In addition, one or more of the following elements are added: Si: 0-2.0%, Cu: 0-2.0wt%, Ni: 0-2.0%, B: 0-0.005wt%, and rare earth elements (RE): 0-0.050wt%. The product has a strength of over 700MPa and a density ≤6.7g / cm³. 3 However, the Mn content is as high as 25-35%, and the aging time is as high as 20-80 hours, which leads to high alloy cost, complex and inefficient production process, making it difficult to meet the demand for cost reduction in production.

[0006] Chinese patent application CN116200678A discloses a "hot-rolled coil for a 700MPa high-strength lightweight photovoltaic bracket and its manufacturing method." The chemical composition and mass percentage of the hot-rolled coil are as follows: C: 0.055%–0.09%, Si: 0.10%–0.35%, Mn: 1.30%–1.7%, S: ≤0.002%, P: ≤0.03%, Al: 0.015%–0.060%, Nb: 0.020–0.050%, Ti: 0.045–0.070%, Cr: 0.2%–0.5%, N ≤0.007%, V: ≤0.020%, with the remainder being Fe and other unavoidable impurities. This method achieves overall weight reduction for photovoltaic brackets and also offers significant advantages in reliability and corrosion resistance. Its product strength reaches over 700MPa, but it does not take into account the issue of product lightweighting, making it difficult to meet customers' demand for lightweighting. Furthermore, it is a hot-rolled plate manufacturing method. Summary of the Invention

[0007] This invention provides a corrosion-resistant 700MPa grade lightweight ferritic steel and its preparation method. Based on traditional cold-rolled ferritic steel, it adds the lightweight element Al and the corrosion-resistant element Cr. The alloy cost of the steel is low, which can meet the requirements of automotive lightweighting and comprehensive performance. It can be industrialized and mass-produced using traditional production lines, and has the advantages of low production cost and stable process.

[0008] To achieve the above objectives, the present invention employs the following technical solution:

[0009] A corrosion-resistant, 700MPa grade lightweight ferritic steel, the chemical composition of which, by mass percentage, contains: C: 0.045%–0.055%, Mn: 0.40%–0.55%, Si: 0.40%–0.55%, Al: 5.0%–6.0%, P≤0.01%, S≤0.01%, N≤0.005%, Nb: 0.2%–0.3%, Ti: 0.2%–0.3%, V: 0.2%–0.3%, Cr: 0.1%–0.5%, with the balance being Fe and unavoidable impurities; the finished steel plate has a yield strength of 600–700MPa, a tensile strength ≥700MPa, an elongation after fracture (A) ≥21%, and a density ≤7.23g / cm³. 3 .

[0010] A method for preparing a corrosion-resistant 700MPa grade lightweight ferritic steel includes converter smelting, continuous casting, hot rolling, pickling and cold rolling, and continuous annealing processes, as detailed below:

[0011] 1) Converter smelting: Steel is smelted in a converter to obtain molten steel that meets the composition requirements; the temperature of the molten steel is controlled between 1625 and 1725℃.

[0012] 2) Continuous casting: Continuous casting is carried out using a continuous casting machine. The casting temperature is 1555~1625℃, the casting machine speed is 3.0~6.0m / min, and the thickness of the continuous casting billet is 70~110mm.

[0013] 3) Hot rolling: The heating temperature is 1145~1160℃, the holding time is 80~180min, the initial rolling temperature is 1045~1060℃, and the final rolling temperature is 855~870℃; laminar flow cooling is adopted after rolling, the cooling rate is >20℃ / s, and the coiling temperature is 640~660℃.

[0014] 4) Pickling and cold rolling: Hot-rolled steel coils are pickled to remove surface oxides before cold rolling; the cold rolling reduction rate is 70% to 80%.

[0015] 5) Continuous annealing: heating rate is 10-20℃ / s, annealing temperature is 805-815℃, annealing holding time is 240-280s, slow cooling temperature is 690-740℃, rapid cooling rate is 20-40℃ / s, rapid cooling temperature is 300-330℃; aging treatment temperature is 290-320℃, aging treatment time is 350-400s, and finally cooling to room temperature at a cooling rate of 10-30℃ / s.

[0016] Compared with the prior art, the beneficial effects of the present invention are:

[0017] (1) The lightweight ferritic steel described in this invention adds lightweight element Al and corrosion-resistant element Cr to the traditional cold-rolled ferritic steel, which reduces the overall weight of the steel plate by about 8%, and can simultaneously meet the requirements of automotive lightweighting and comprehensive performance.

[0018] (2) The chemical composition of the lightweight ferritic steel described in this invention is mainly composed of C, Mn, Si, Al and Cr, and the alloy cost of the raw materials is relatively low.

[0019] (3) The present invention adopts a production process of converter smelting-continuous casting-hot rolling-pickling and cold rolling-continuous annealing, which can directly utilize traditional production lines to achieve industrial mass production without the need for additional equipment investment, and has the advantages of low production cost and stable process. Attached Figure Description

[0020] Figure 1 This is the engineering stress-strain curve of the 700MPa grade lightweight ferritic steel in Embodiment 1 of the present invention. Detailed Implementation

[0021] The present invention discloses a corrosion-resistant 700MPa grade lightweight ferritic steel. The chemical composition of the steel, by mass percentage, contains: C: 0.045%–0.055%, Mn: 0.40%–0.55%, Si: 0.40%–0.55%, Al: 5.0%–6.0%, P≤0.01%, S≤0.01%, N≤0.005%, Nb: 0.2%–0.3%, Ti: 0.2%–0.3%, V: 0.2%–0.3%, Cr: 0.1%–0.5%, with the balance being Fe and unavoidable impurities. The finished steel plate has a yield strength of 600–700MPa, a tensile strength ≥700MPa, an elongation after fracture (A) ≥21%, and a density ≤7.23g / cm³. 3 .

[0022] The alloy composition and design rationale for the corrosion-resistant 700MPa grade lightweight ferritic steel described in this invention are as follows:

[0023] C: Carbon strengthens steel through solid solution treatment to ensure its strength requirements. Sufficient carbon content helps stabilize austenite, thereby improving the formability of the steel. Too low a carbon content increases the difficulty of controlling the smelting process. Too high a carbon content affects the surface quality of the steel plate and deteriorates weldability. Therefore, this invention controls the carbon content to be between 0.045% and 0.055%.

[0024] Manganese (Mn) enhances the strength of steel through solid solution strengthening and stabilizes austenite, thereby improving the steel's work hardening ability. Simultaneously, manganese can mitigate or eliminate hot brittleness caused by sulfur (S), thus improving the steel's hot workability. If the manganese content is too low, the stability of supercooled austenite decreases, leading to weakened processing properties such as plasticity and toughness in the steel plate. If the Mn content is too high, it may cause the formation of banded structures in the steel plate, impairing its toughness and deteriorating its weldability, while also increasing production costs, which is detrimental to large-scale industrial production. Therefore, this invention controls the Mn content to be between 0.40% and 0.55%.

[0025] Silicon (Si): As a ferrite solid solution strengthening element, silicon can significantly improve the strength of steel. Simultaneously, Si can inhibit carbide precipitation and reduce inclusions in steel. If the Si content is too low, it will not achieve the strengthening and carbide-inhibiting effects; if the Si content is too high, it will reduce the plasticity and weldability of the steel plate. Therefore, this invention controls the Si content at 0.40%–0.55%.

[0026] Al: Al has a certain solid solution strengthening effect in ferrite, which can significantly improve the strength of steel. At the same time, Al can effectively reduce the density of steel. If the Al content is too low, the purpose of reducing the weight of steel will not be achieved; if the Al content is too high, it will affect the stability of austenite, thereby reducing the formability of the steel plate. Therefore, this invention controls the Al content at 5.0% to 6.0%.

[0027] Phosphorus (P): Phosphorus has a certain solid solution strengthening effect, which can enhance the strength of steel. However, the presence of phosphorus also increases the cold brittleness of steel and reduces its plasticity and formability. Considering cost, this invention controls the P content to P≤0.01%.

[0028] Sulfur (S): Sulfur is a harmful element in steel. It readily combines with manganese to form MnS inclusions, which weaken the ductility and toughness of the steel. Therefore, the sulfur content should be reduced as much as possible to optimize the performance of the steel. Considering cost, this invention controls the S content to S≤0.01%.

[0029] Nitrogen (N): Nitrogen readily reacts with Ti and Al to produce large TiN and AlN particles. These coarse TiN and AlN particles negatively impact the ductility of the steel sheet. Considering cost, this invention controls the N content to ≤0.005%.

[0030] Nb: Adding an appropriate amount of niobium to steel can significantly suppress grain coarsening during hot working by forming Nb(C,N) compounds, while also hindering the dynamic recrystallization process to improve rolling deformation resistance and promote ferrite grain refinement. In this invention, the Nb content is controlled at 0.2% to 0.3%.

[0031] Ti: Titanium combines with carbon and nitrogen to form compounds such as Ti(C,N), TiN, and TiC, which can effectively improve the as-cast grain structure and inhibit grain growth during high-temperature processing. However, excessive addition can significantly increase production costs, and excessive enrichment of second-phase particles may lead to a decrease in the material's ductility. Therefore, this invention controls the Ti content to 0.2%–0.3%.

[0032] Vanadium (V): The microalloying element vanadium can promote grain refinement in steel, while simultaneously improving material stability by suppressing grain coarsening at high temperatures, thus giving the steel plate better strength and plasticity. Therefore, the V content in this invention is controlled at 0.2% to 0.3%.

[0033] Cr: Chromium is a ferrite-forming element that stabilizes ferrite. Furthermore, chromium atoms incorporated into the body-centered cubic (BCC) structure of ferrite cause lattice distortion, thereby increasing the strength and hardness of the steel sheet, but slightly affecting its plasticity. Additionally, chromium refines ferrite grains, particularly inhibiting grain growth at high temperatures. Therefore, this invention controls the Cr content to 0.1%–0.5%.

[0034] The present invention discloses a method for preparing a corrosion-resistant 700MPa grade lightweight ferritic steel, comprising converter smelting, continuous casting, hot rolling, pickling and cold rolling, and continuous annealing processes, as detailed below:

[0035] 1) Converter smelting: Steel is smelted in a converter to obtain molten steel that meets the composition requirements; the temperature of the molten steel is controlled between 1625 and 1725℃.

[0036] 2) Continuous casting: Continuous casting is carried out using a continuous casting machine. The casting temperature is 1555~1625℃, the casting machine speed is 3.0~6.0m / min, and the thickness of the continuous casting billet is 70~110mm.

[0037] 3) Hot rolling: Heating temperature is 1145–1160℃, holding time is 80–180 min, initial rolling temperature is 1045–1060℃, and final rolling temperature is 855–870℃; laminar flow cooling is used after rolling, with a cooling rate >20℃ / s, and coiling temperature is 640–660℃. Hot rolling uses single-phase rolling, utilizing medium-temperature final rolling and medium-temperature coiling to ensure uniform grain growth, which improves both the stamping performance and strength of the steel plate. Excessively high final rolling temperature leads to coarse grains, while excessively low final rolling temperature leads to entry into the two-phase region, resulting in abnormal grain growth in the microstructure. Using medium-temperature coiling ensures uniform grain growth in the steel; if the coiling temperature is too low, the grains will be too small, leading to a decrease in the elongation of the steel plate.

[0038] 4) Pickling and cold rolling: Before cold rolling, hot-rolled steel coils are pickled to remove surface oxides; the cold rolling reduction rate is 70% to 80%; using a high cold rolling reduction rate can enhance the γ-fiber texture of the steel plate, which is beneficial to improving the deep drawing performance of the steel plate.

[0039] 5) Continuous annealing: The heating rate is 10–20℃ / s, the annealing temperature is 805–815℃, and the annealing holding time is 240–280s. Excessive annealing temperature or time will result in coarse austenite grains, reducing the strength and elongation of the steel plate. Insufficient annealing temperature or time will prevent complete recrystallization of ferrite, resulting in insufficient grain growth and affecting the elongation of the steel plate. Slow cooling temperature is 690–740℃, rapid cooling rate is 20–40℃ / s, and rapid cooling temperature is 300–330℃. A cooling temperature between 300 and 330℃ ensures sufficient precipitation and agglomeration of dissolved carbon in the steel, which is beneficial for improving elongation, mechanical properties, and stamping performance of the steel plate. Insufficient temperature will lead to insufficient precipitation of dissolved carbon, reducing the elongation and stamping performance of the steel plate. The aging treatment temperature is 290–320℃, and the aging time is 350–400 s. A short aging time will result in insufficient precipitation of dissolved carbon, affecting mechanical properties; an excessively long time will lead to coarsening of the precipitated phase and reduced strength. Finally, the aging is cooled to room temperature at a cooling rate of 10–30℃ / s.

[0040] To more intuitively illustrate the present invention, the embodiments of the present invention will be further described in conjunction with the examples. The following examples are merely preferred embodiments of the present invention, but the scope of protection of the present invention is not limited thereto. Any technical solutions that can be obviously obtained by those skilled in the art within the scope of the technology disclosed in the present invention, including simple variations or equivalent substitutions, are all within the scope of protection of the present invention.

[0041] Example:

[0042] The chemical composition of the steel in each embodiment is shown in Table 1, the continuous casting process parameters of the steel in each embodiment are shown in Table 2, the hot rolling process parameters of the steel plate in each embodiment are shown in Table 3, the continuous annealing process parameters of the steel plate in each embodiment are shown in Table 4, and the mechanical properties of the finished steel plate in each embodiment are shown in Table 5. Figure 1 The stress-strain curve is the engineering stress-strain curve of the steel plate produced in Example 1.

[0043] Table 1 Chemical composition of steel, wt%

[0044]

[0045] Table 2 Continuous casting process parameters for steel

[0046]

[0047] Table 3 Hot rolling process parameters for steel plates

[0048]

[0049] Table 4 Continuous Annealing Process Parameters for Steel Plates

[0050]

[0051] Table 5 Mechanical Properties of Finished Steel Plates

[0052]

[0053] As can be seen from the above embodiments, the corrosion-resistant lightweight ferritic steel plate prepared using the composition design, continuous casting, hot rolling, and continuous annealing process of the present invention has a yield strength of 600-700 MPa, a tensile strength ≥700 MPa, an elongation after fracture A ≥21%, and a density ≤7.23 g / cm³. 3 This meets the requirements for lightweighting and good stamping performance in automobiles.

[0054] 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 corrosion-resistant 700MPa grade lightweight ferritic steel, characterized in that, The chemical composition of the steel, by mass percentage, contains: C: 0.045%–0.055%, Mn: 0.40%–0.55%, Si: 0.40%–0.55%, Al: 5.0%–6.0%, P≤0.01%, S≤0.01%, N≤0.005%, Nb: 0.2%–0.3%, Ti: 0.2%–0.3%, V: 0.2%–0.3%, Cr: 0.1%–0.5%, with the balance being Fe and unavoidable impurities; The finished steel plate has a yield strength of 600–700 MPa, a tensile strength of ≥700 MPa, an elongation after fracture (A) of ≥21%, and a density of ≤7.23 g / cm³. 3 .

2. A method for preparing the corrosion-resistant 700MPa grade lightweight ferritic steel as described in claim 1, characterized in that, The processes include converter smelting, continuous casting, hot rolling, pickling and cold rolling, and continuous annealing, as detailed below: 1) Converter smelting: Steel that meets the composition requirements is smelted in a converter. 2) Continuous casting: Continuous casting is carried out using a continuous casting machine to obtain continuously cast billets; 3) Hot rolling: The heating temperature is 1145~1160℃, the initial rolling temperature is 1045~1060℃, and the final rolling temperature is 855~870℃; laminar flow cooling is adopted after rolling, the cooling rate is >20℃ / s, and the coiling temperature is 640~660℃. 4) Pickling and cold rolling: Hot-rolled steel coils are pickled to remove surface oxides before cold rolling; the cold rolling reduction rate is 70% to 80%. 5) Continuous annealing: heating rate is 10-20℃ / s, annealing temperature is 805-815℃, annealing holding time is 240-280s, slow cooling temperature is 690-740℃, rapid cooling rate is 20-40℃ / s, rapid cooling temperature is 300-330℃; aging treatment temperature is 290-320℃, aging treatment time is 350-400s, and finally cooling to room temperature at a cooling rate of 10-30℃ / s.

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