High-temperature-resistant 800mpa-grade lightweight ferrite steel and preparation method thereof

Abstract

The present application relates to the technical field of cold-rolled automobile steel, and particularly relates to a high-temperature-resistant 800MPa-grade lightweight ferrite steel and a preparation method thereof, wherein the chemical components of the steel are as follows: C: 0.055%-0.065%, Mn: 0.55%-0.70%, Si: 0.55%-0.70%, Al: 6.0%-7.0%, P: 0.01% or less, S: 0.01% or less, N: 0.005% or less, Nb: 0.3%-0.4%, Ti: 0.3%-0.4%, V: 0.3%-0.4%, W: 0.1%-0.5%, and the balance is Fe and impurities. On the basis of the traditional cold-rolled ferrite steel, the lightweight element Al and the high-temperature-resistant and wear-resistant element W are added, so that the overall weight of the steel plate is reduced by about 10%, and the light weight and the comprehensive performance of the automobile are met.

Description

Technical Field

[0001] This invention relates to the field of cold-rolled automotive steel technology, and in particular to a high-temperature resistant 800MPa 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 high-temperature resistant 800MPa grade lightweight ferritic steel that can be produced on a traditional cold rolling production line at extremely low alloy cost. It can meet the requirements of low cost, high lightweight and corrosion resistance, and provide a feasible technical solution for automobile manufacturers and steel companies.

[0004] Chinese invention patent CN104928569B discloses "an 800MPa grade high-ductility low-density steel and its manufacturing method." The main chemical composition of the steel is: C: 0.25-0.50%, Mn: 0.25-4.0%, Al: 3.0-7.0%, P≤0.02%, S≤0.01%, N≤0.01%, with the remainder being Fe and unavoidable impurities. Furthermore, the above elements must simultaneously satisfy the following relationship: 0.14≤(1.5C+0.1Mn) / Al≤0.30. While the steel plate achieves a strength of over 800MPa and also considers vehicle lightweighting, its density is <7.5g / cm³, resulting in a weight reduction of approximately 5%. This is not a significant achievement in vehicle lightweighting and fails to meet the requirements for energy conservation and emission reduction.

[0005] Chinese invention patent CN104928568B discloses "A ferritic low-density high-strength steel and its manufacturing method." The main chemical composition of the steel is: C: 0.05-0.40%, Mn: 4.0-12.0%, Al: 3.0-7.0%, P≤0.02%, S≤0.01%, N≤0.01%, with the balance being Fe and other unavoidable impurities. The steel plate achieves a strength of 800 MPa, but the excessively high Mn content poses difficulties for actual production. Furthermore, its chemical composition results in high costs and a complex production process, making it difficult to meet the demand for cost reduction in production. Summary of the Invention

[0006] This invention provides a high-temperature resistant 800MPa grade lightweight ferritic steel and its preparation method. Based on traditional cold-rolled ferritic steel, it adds the lightweight element Al and the high-temperature resistant and wear-resistant element W, which reduces the overall weight of the steel plate by about 10%, meeting the requirements of automobiles for lightweighting and comprehensive performance. The chemical composition is mainly composed of C, Mn, Si, Al and W, with low raw material costs, and industrial mass production can be achieved using traditional production lines.

[0007] To achieve the above objectives, the present invention employs the following technical solution:

[0008] A high-temperature resistant (800MPa) lightweight ferritic steel, comprising the following chemical composition by mass percentage: C: 0.055%–0.065%, Mn: 0.55%–0.70%, Si: 0.55%–0.70%, Al: 6.0%–7.0%, P≤0.01%, S≤0.01%, N≤0.005%, Nb: 0.3%–0.4%, Ti: 0.3%–0.4%, V: 0.3%–0.4%, W: 0.1%–0.5%, with the balance being Fe and unavoidable impurities. The steel exhibits a yield strength of 700–800MPa, a tensile strength ≥800MPa, an elongation at break (A) ≥19%, and a density ≤7.11g / cm³. 3 .

[0009] A method for preparing a high-temperature resistant 800MPa lightweight ferritic steel includes converter smelting, continuous casting, hot rolling, pickling and cold rolling, and continuous annealing processes, as detailed below:

[0010] 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 1630 and 1730℃.

[0011] 2) Continuous casting: Continuous casting is carried out using a continuous casting machine. The casting temperature is 1560~1630℃, the casting speed is 3.0~6.0m / min, and the thickness of the continuously cast billet is 70~110mm.

[0012] 3) Hot rolling: The heating temperature is 1130~1145℃, the holding time is 85~185min, the initial rolling temperature is 1030~1045℃, and the final rolling temperature is 840~855℃; laminar flow cooling is adopted after rolling, the cooling rate is >20℃ / s, and the coiling temperature is 620~640℃.

[0013] 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%.

[0014] 5) Continuous annealing: heating rate is 10-20℃ / s, annealing temperature is 795-805℃, annealing holding time is 280-320s, slow cooling temperature is 690-720℃, rapid cooling rate is 20-40℃ / s, rapid cooling temperature is 270-300℃; aging treatment temperature is 260-290℃, aging treatment time is 400-450s, and finally cooling to room temperature at a cooling rate of 10-30℃ / s.

[0015] Compared with the prior art, the beneficial effects of the present invention are:

[0016] (1) The main chemical components of steel are C, Mn, Si, Al and W, and the raw material cost is relatively low.

[0017] (2) The production process of converter smelting-continuous casting-hot rolling-pickling and cold rolling-continuous annealing can be directly realized on the existing production line for industrial mass production without the need for additional equipment investment, and has the significant advantages of low production cost and stable process.

[0018] (3) The 800MPa grade lightweight ferritic steel produced is a low-density ferritic steel. It adds lightweight element Al and high-temperature and wear-resistant element W on the basis of traditional cold-rolled ferritic steel, which reduces the overall weight of the steel plate by about 10%, meeting the requirements of automobile lightweighting and comprehensive performance. Attached Figure Description

[0019] Figure 1 This is the engineering stress-engineering strain curve of the finished steel plate in Embodiment 1 of the present invention. Detailed Implementation

[0020] The present invention discloses a high-temperature resistant 800MPa grade lightweight ferritic steel. The chemical composition of the steel, by mass percentage, is as follows: C: 0.055%–0.065%, Mn: 0.55%–0.70%, Si: 0.55%–0.70%, Al: 6.0%–7.0%, P≤0.01%, S≤0.01%, N≤0.005%, Nb: 0.3%–0.4%, Ti: 0.3%–0.4%, V: 0.3%–0.4%, W: 0.1%–0.5%, with the balance being Fe and unavoidable impurities. The steel has a yield strength of 700–800MPa, a tensile strength ≥800MPa, an elongation at break (A) ≥19%, and a density ≤7.11g / cm³. 3 .

[0021] The alloy composition and design rationale for the high-temperature resistant 800MPa grade lightweight ferritic steel described in this invention are as follows:

[0022] 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 0.055% and 0.065%.

[0023] 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 the 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 0.55% and 0.70%.

[0024] 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 to be between 0.55% and 0.70%.

[0025] 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 6.0% to 7.0%.

[0026] 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%.

[0027] 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%.

[0028] 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%.

[0029] Nb: Adding an appropriate amount of niobium to steel can significantly suppress grain coarsening during hot working by forming Nb(C,N) compounds, while hindering the dynamic recrystallization process to improve rolling deformation resistance and promote ferrite grain refinement. In this invention, the Nb content is controlled at 0.3% to 0.4%.

[0030] 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 0.3%–0.4%.

[0031] 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 0.3% to 0.4%.

[0032] Tungsten (W) exists in ferrite in a substitutional solid solution form, causing lattice distortion and hindering dislocation movement, thereby increasing the strength and hardness of ferrite. Simultaneously, W combines with carbon to form stable carbides (such as W₂C, Fe₃W₃C, or complex carbides). These hard particles distributed in the ferrite matrix enhance the material's wear resistance and hardness. Furthermore, the precipitation of these carbides at grain boundaries can prevent grain coarsening, refine the ferrite structure, and thus improve the material's toughness. However, tungsten is expensive; considering cost, this invention controls the W content to 0.1%–0.5%.

[0033] The present invention discloses a method for preparing a high-temperature resistant 800MPa grade lightweight ferritic steel, comprising converter smelting, continuous casting and rolling, pickling and cold rolling, and continuous annealing processes, as detailed below:

[0034] 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 1630 and 1730℃.

[0035] 2) Continuous casting: Continuous casting is carried out using a continuous casting machine. The casting temperature is 1560~1630℃, the casting speed is 3.0~6.0m / min, and the thickness of the continuously cast billet is 70~110mm.

[0036] 3) Hot rolling: Heating temperature is 1130–1145℃, holding time is 85–185 min, initial rolling temperature is 1030–1045℃, and final rolling temperature is 840–855℃; laminar flow cooling is used after rolling, with a cooling rate >20℃ / s, and coiling temperature is 620–640℃; 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. Medium-temperature coiling ensures sufficient 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.

[0037] 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.

[0038] 5) Continuous annealing: The heating rate is 10–20℃ / s, the annealing temperature is 795–805℃, and the annealing holding time is 280–320s. 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 690–720℃, rapid cooling rate is 20–40℃ / s, and rapid cooling temperature is 270–300℃. A cooling temperature between 270 and 300℃ 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 260–290℃, and the aging treatment time is 400–450 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.

[0039] 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.

[0040] Example:

[0041] 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.

[0042] Table 1. Chemical composition of steel, wt%

[0043]

[0044] Table 2 Continuous casting process parameters for steel

[0045]

[0046] Table 3 Hot rolling process parameters for steel plates

[0047]

[0048] Table 4 Continuous Annealing Process Parameters for Steel Plates

[0049]

[0050] Table 5 Mechanical Properties of Finished Steel Plates

[0051]

[0052] 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 processes of the present invention has a yield strength of 700-800 MPa, a tensile strength ≥800 MPa, an elongation after fracture A ≥19%, and a density ≤7.11 g / cm³. 3 This meets the requirements for lightweighting and good stamping performance in automobiles.

[0053] 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-temperature resistant 800MPa grade lightweight ferritic steel, characterized in that, The chemical composition of the steel, by mass percentage, contains: C: 0.055%–0.065%, Mn: 0.55%–0.70%, Si: 0.55%–0.70%, Al: 6.0%–7.0%, P≤0.01%, S≤0.01%, N≤0.005%, Nb: 0.3%–0.4%, Ti: 0.3%–0.4%, V: 0.3%–0.4%, W: 0.1%–0.5%, with the balance being Fe and unavoidable impurities. The steel has a yield strength of 700–800 MPa, a tensile strength ≥800 MPa, an elongation at fracture (A) ≥19%, and a density ≤7.11 g / cm³. 3 .

2. A method for preparing the high-temperature resistant 800MPa 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 is smelted in a converter to obtain molten steel that meets the composition requirements; the temperature of the molten steel is controlled between 1630 and 1730℃. 2) Continuous casting: Continuous casting is carried out using a continuous casting machine. The casting temperature is 1560~1630℃, the casting speed is 3.0~6.0m / min, and the thickness of the continuously cast billet is 70~110mm. 3) Hot rolling: The heating temperature is 1130~1145℃, the holding time is 85~185min, the initial rolling temperature is 1030~1045℃, and the final rolling temperature is 840~855℃; laminar flow cooling is adopted after rolling, the cooling rate is >20℃ / s, and the coiling temperature is 620~640℃. 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 795-805℃, annealing holding time is 280-320s, slow cooling temperature is 690-720℃, rapid cooling rate is 20-40℃ / s, rapid cooling temperature is 270-300℃; aging treatment temperature is 260-290℃, aging treatment time is 400-450s, and finally cooling to room temperature at a cooling rate of 10-30℃ / s.

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