500MPa-grade cold-rolled high-strength outer plate steel for automobile based on rapid heating treatment technology and preparation method of 500MPa-grade cold-rolled high-strength outer plate steel

Abstract

The invention belongs to the field of automobile steel manufacturing, and particularly relates to 500 MPa grade automobile cold-rolled high-strength outer plate steel based on a rapid heating treatment technology and a preparation method of the 500 MPa grade automobile cold-rolled high-strength outer plate steel. Mn: 0.6% to 1.50%; 0.10%-0.7% of Si; p is less than or equal to 0.030%; n < = 0.002%; s < = 0.005%; the content of Nb, Ti and V is more than or equal to 0.01% and less than or equal to 0.02%; and the balance of Fe and inevitable impurities. The method has the advantages that the purpose of grain strengthening is achieved by adopting a rapid heating continuous annealing process, the strength of the steel for the automobile outer plate is improved, and meanwhile excellent forming performance is achieved.

Description

Technical Field

[0001] This invention belongs to the field of automotive steel manufacturing, and particularly relates to a 500MPa grade cold-rolled high-strength outer steel for automotive use based on rapid heat treatment technology and its preparation method. Background Technology

[0002] Lightweighting of automobiles can effectively save energy and reduce emissions, and is a major trend in automobile body development. As the most important material in automobiles, high-strength steel can not only meet the requirements of lightweighting but also effectively improve the safety performance of vehicles. Therefore, increasing the strength of steel used in automobiles is the development direction of steel materials. Currently, the main steel products used in automotive outer panels are interstitial (IF) steel and bake-hardening (BH) steel, and their mainstream tensile strength is 340 MPa and below. Therefore, developing higher-strength steel for automotive outer panels will contribute to the realization of automobile lightweighting.

[0003] Patent CN103305757A discloses a high-strength automotive steel sheet, produced using processes such as smelting, hot rolling, cold rolling, and continuous annealing to create a 340MPa-grade high-strength steel product that combines high strength with good formability. However, this product's strength level is comparable to commonly used automotive outer panel steel and cannot further meet the demands of lightweight automotive applications.

[0004] Patent CN115181894A discloses a 590MPa grade high-formability hot-dip galvanized duplex steel and a rapid heat treatment hot-dip galvanizing manufacturing method. The steel composition by mass percentage is: C: 0.045~0.105%, Si: 0.1~0.4%, Mn: 1.0~1.5%, P≤0.02%, S≤0.006%, Al: 0.02~0.055%, and may also contain one or two of Cr, Mo, Ti, Nb, and V, with Cr+Mo+Ti+Nb+V≤0.3%, and the balance being Fe and other unavoidable impurities. The hot-dip galvanizing steps include: rapid heating~short holding~rapid cooling~hot-dip galvanizing~rapid cooling (hot-dip pure zinc GI product); rapid heating~short holding~rapid cooling~hot-dip galvanizing~reheating~alloying treatment~rapid cooling (alloyed hot-dip galvanized GA product). The ferrite and martensite dual-phase structure obtained by the above composition and process results in a steel plate with a yield strength of 347–405 MPa, a tensile strength of 624–709 MPa, an elongation of 22.2–30.3%, and a strength-ductility product of 15.7–19.6 GPa%. However, the yield strength and elongation of this steel plate are not suitable for cold stamping of complex automotive body panels.

[0005] As is well known, the properties of materials are closely related to their microstructure, and the various properties of steel materials are generally determined by the type, content, and size of their phase structure (ferrite, bainite, martensite, and austenite, etc.). Compared with heat treatment processes with conventional heating rates, rapid heating can significantly affect the recovery and recrystallization stages of cold-rolled structures, effectively promoting microstructure refinement, which is beneficial for improving strength and plasticity. Simultaneously, it can rapidly increase production efficiency and reduce production costs. Therefore, the continuous annealing heat treatment technology using rapid heating can be used to develop high-strength steels for automobiles and has broad application prospects. Summary of the Invention

[0006] To overcome the shortcomings of the prior art, the purpose of this invention is to provide a 500MPa grade cold-rolled high-strength steel for automotive outer panels based on rapid heat treatment technology and its preparation method. Through the rational design of composition and production process based on rapid heat treatment technology, a high-strength steel for outer panels with excellent formability is obtained.

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

[0008] A 500MPa grade cold-rolled high-strength steel for automotive outer panels based on rapid heat treatment technology, the chemical composition of which is as follows by weight percentage:

[0009] C: 0.05%~0.11%; Mn: 0.6%~1.50%; Si: 0.10%~0.7%; P≤0.030%; N≤0.002%; S≤0.005%; one or more of Nb, Ti and V, and the content of Nb+Ti+V ≤0.02% ≤0.01%; the balance is Fe and unavoidable impurities.

[0010] The microstructure of the steel plate includes: critical zone ferrite, oriented epiphytic ferrite, bainite, martensite, secondary martensite, and retained austenite. The volume percentages of each microstructure are as follows: critical zone ferrite: 55%–70%, oriented epiphytic ferrite: 5%–15%, bainite + martensite: 8%–13%, secondary martensite: 5%–10%, and retained austenite: 5%–12%. The grain size of the ferrite is 2–12 μm.

[0011] The steel plate has a tensile strength ≥500MPa, a yield strength of 250~310MPa, and an elongation after fracture of 30%~36%.

[0012] The rationale for the alloy design of this invention is as follows:

[0013] Carbon (C): Carbon is the most abundant element in steel, primarily existing as interstitial atoms. It is the most effective solid solution strengthening element and an indispensable element in steel. In this invention, carbon also plays a role in stabilizing austenite. During heat treatment, carbon can rapidly diffuse from ferrite to the austenite phase, allowing partial retention of austenite in the subsequent rapid cooling and aging stages, thus improving the plasticity of the steel plate. However, excessive carbon content can lead to excessively high strength, reduced plasticity and toughness, and negatively impact the surface quality of the steel plate, while also deteriorating its weldability. Therefore, the carbon content is required to be between 0.05% and 0.11%.

[0014] Mn: Mn is a common additive element in steel, existing in the form of substitutional atoms, and is one of the main strengthening elements. At the same time, Mn is also an important temperature austenite element, which can delay the transformation of pearlite, thus contributing to obtaining the ideal microstructure required in this invention. Mn diffuses much slower than carbon, resulting in localized aggregation in steel. During the heat treatment stage of this invention, rapid heating promotes the segregation of Mn, which is beneficial for the retention of residual austenite and improves plasticity. Excessive Mn content easily leads to oxidation of the steel plate during hot rolling and continuous annealing, reducing the surface quality of the steel plate and also increasing the carbon equivalent, severely affecting welding quality. Therefore, this invention requires an Mn content of 0.6% to 1.50%.

[0015] Si: Si is also an important additive element, strengthening ferrite and improving the overall strength of the steel plate. In addition, Si promotes the diffusion of C and Mn into the austenite phase, and effectively inhibits carbide precipitation during heat treatment, thus stabilizing the austenite phase and ensuring the acquisition of residual austenite. However, excessive Si content reduces the weldability of the steel plate, which is detrimental to subsequent applications. Therefore, this invention requires a Si content of 0.1% to 0.7%.

[0016] P: P is a common element in steel. This element easily forms grain boundary segregation, drastically reducing the plasticity of the steel sheet and affecting its formability. Therefore, this invention requires that the P content be ≤0.030%.

[0017] N: N is a harmful element in steel, and the lower its content, the better. Therefore, this invention requires that the N content be ≤0.002%.

[0018] S: S is a harmful element in steel, which easily reacts with Mn to form MnS, creating inclusions and affecting various properties of the steel plate. Therefore, its content should be as low as possible. Thus, this invention requires the S element content to be ≤0.005%.

[0019] Ti, Nb, and V: The microalloying elements Ti, Nb, and V primarily serve to refine the grain size. These three microalloying elements readily form fine precipitates with elements such as C and N in steel, which can pin grain boundaries during hot rolling and continuous annealing, thus refining the grain size. Furthermore, Nb itself can also act as a solute atom dragger, effectively refining the grain size and providing strengthening. Therefore, this invention requires the addition of one or more of Ti, Nb, and V elements, and requires that the content of Nb+Ti+V ≤ 0.01% ≤ 0.02%.

[0020] A method for manufacturing 500MPa grade cold-rolled high-strength steel for automotive outer plates based on rapid heat treatment technology includes smelting, hot rolling, pickling, cold rolling, continuous annealing, and finishing.

[0021] Smelting: The alloy composition within the above range is obtained by smelting in a converter or refining furnace, and then cast into a billet.

[0022] Hot-rolled:

[0023] 1) Heating temperature: 1200–1280℃, furnace time: ≥120min. Heating temperatures above 1200℃ ensure uniform diffusion of elements in the steel and eliminate microscopic defects, resulting in a uniform microstructure. However, temperatures exceeding 1280℃ can lead to excessively large grains and overheating, affecting the final properties of the steel plate. Therefore, a heating temperature of 1200–1280℃ and a heating time of ≥120min guarantee uniform distribution of alloying elements.

[0024] 2) Rough rolling temperature 1030~1100℃, final rolling temperature ≥880℃. This ensures that the steel plate is rolled in the austenitic single-phase range, resulting in dynamic recrystallization, which effectively refines the grains and eliminates the appearance of mixed grain structures.

[0025] 3) Intermediate cooling temperature 700-730℃, and subsequent cooling rate >60℃ / s; a two-stage cooling mode is adopted after finishing rolling. The intermediate cooling temperature is 700-730℃, which can make the precipitates containing Ti, Nb or V completely dispersed and fine, which is conducive to the uniformity and fineness of the final microstructure. At the same time, the subsequent cooling rate >60℃ / s also inhibits the growth of ferrite grains and the aggregation and growth of precipitates, which also ensures the fineness of the microstructure, improves the uniformity of the material structure, and is conducive to the improvement of the strength and plasticity of the material.

[0026] 4) The coiling temperature is 580-670℃, which refines the ferrite grains of the steel plate, mainly forming ferrite and pearlite structures, while avoiding the increase in the difficulty of cold rolling due to bainite or martensite phase transformation caused by the low temperature.

[0027] Pickling: Removes iron oxides from the surface of the steel sheet after hot rolling and coiling, ensuring the surface quality of the cold-rolled sheet.

[0028] Cold rolling:

[0029] The cold rolling reduction rate is controlled between 55% and 70% to ensure sufficient energy storage during cold rolling deformation. This provides the driving force for the recovery and recrystallization of the microstructure in the subsequent annealing process, ensuring grain refinement and thus enabling the steel sheet to achieve the target strength and plasticity. However, an excessively high rolling reduction rate will increase the load on the cold rolling mill and may not guarantee the achievement of the target thickness.

[0030] Continuous annealing:

[0031] 1) During annealing, the heating rate is 30–120℃ / s, the isothermal temperature is 760–850℃, and the annealing time is 5–20s. In continuous annealing, the cold-rolled fiber structure undergoes recovery, recrystallization, grain growth, and phase transformation. Increasing the heating rate affects the recovery and recrystallization of the deformed structure and raises the transformation temperature from ferrite to austenite. During rapid heating, the deformed structure retains a large number of dislocations after recovery, providing more nucleation sites for recrystallization and refining the structure. Simultaneously, rapid heating raises the recrystallization completion temperature, meaning that phase transformation may begin when the deformed structure has completed recrystallization and grain growth. Therefore, the numerous defects in the ferrite grains, combined with the fine ferrite grains, provide more nucleation sites for the transformation from ferrite to austenite, increasing the nucleation rate. Thus, rapid heating can effectively refine the grains. Annealing isothermal temperatures of 760–850℃ ensure that the cold-rolled deformation structure of the steel plate completes recrystallization, forming a favorable recrystallization {111} texture. Simultaneously, it causes the dissolution of pearlite in the hot-rolled structure, placing the structure in a two-phase region of austenite and ferrite. Furthermore, the ferrite content is relatively high within this temperature range, achieving the critical region ferrite content (55%–70%). Annealing times of 5–20 seconds ensure the formation of the favorable ferrite recrystallization texture, while also guaranteeing sufficient diffusion of carbon from ferrite to the austenite phase. Additionally, it allows a concentration gradient of austenite-stabilizing elements such as Mn within the austenite phase, facilitating the retention of some residual austenite during subsequent cooling. However, excessively long annealing times result in overly large austenite grains, reducing the final strength of the steel plate.

[0032] 2) The slow cooling rate is 2-8℃ / s and the slow cooling temperature is 680-720℃. The slow cooling rate of 2-8℃ / s and the slow cooling temperature of 680-720℃ cause the steel plate to undergo ferrite phase transformation during the slow cooling stage, generating 5-15% oriented epiphytic ferrite. At the same time, the carbon element in the ferrite diffuses into the austenite, further stabilizing the austenite.

[0033] 3) After slow cooling, the temperature is reduced to an aging temperature of 380–430℃ at a rapid cooling rate of >18℃ / s, and the aging time is 30–100s. The rapid cooling rate of >18℃ / s can cause the supercooled austenite in the steel plate to undergo martensitic or bainitic phase transformation. The aging temperature of 380–430℃ and the aging time of 30–100s can cause the generated martensite to undergo a tempering reaction, improving the plasticity of the steel plate. At the same time, during this stage, the carbon elements in the tempered martensite and bainite are further enriched into the austenite, improving its stability and achieving the final microstructure.

[0034] 4) After the aging stage, the temperature is reduced to room temperature at a final cooling rate of >10℃ / s. During this stage, the unstable part of the supercooled austenite undergoes martensitic transformation, thereby forming 5% to 10% secondary martensite and 5% to 12% retained austenite, which improves the strength and plasticity of the steel plate.

[0035] Finishing:

[0036] The steel plate enters the finishing machine for shape adjustment, eliminating the yield plateau, and the finishing elongation is 0.2%~0.8%.

[0037] Compared with the prior art, the beneficial effects of the present invention are:

[0038] 1. This invention employs a rapid heating continuous annealing process to achieve grain strengthening, thereby improving the strength of steel used in automotive outer panels and achieving excellent formability. It is particularly suitable for automotive parts such as doors, hoods, and trunk lids.

[0039] 2. This invention uses low-cost C, Mn and Si materials, which has the advantage of low cost.

[0040] 3. The microstructure of the steel plate of the present invention is composed of 55% to 70% critical zone ferrite, 5% to 15% oriented epiphytic ferrite, 8% to 13% bainite + tempered martensite, 5% to 12% retained austenite and 5% to 10% secondary martensite.

[0041] 4. This invention achieves excellent strength and formability of the steel plate through low-cost alloy design and ingenious process design. The tensile strength of the steel plate is ≥500MPa, the yield strength is 250~310MPa, and the elongation after fracture is 30%~36%. Detailed Implementation

[0042] The present invention will be described in more detail through embodiments. These embodiments are merely descriptions of the best mode of the invention and do not limit the scope of the invention in any way.

[0043] Table 1 lists the chemical composition of the steels in the examples, Table 2 lists the hot rolling and cold rolling process parameters of the steels in the examples, Table 3 lists the continuous annealing process parameters of the steels in the examples, and Table 4 gives the mechanical properties of the steels in the examples.

[0044] Table 1 Chemical composition of the steel in the examples, wt%

[0045]

[0046] Table 2 Hot rolling and cold rolling processes for steel in the examples

[0047]

[0048] Table 3. Annealing process for steel in the examples

[0049]

[0050] Table 4. Phase proportions of the embodiments

[0051]

[0052] Table 5 Mechanical properties of the steel in the examples

[0053]

[0054] As can be seen from the above embodiments, by combining low-cost composition design with an innovative rapid heating and continuous annealing process, a 500MPa grade cold-rolled high-strength steel for automotive outer panels was prepared. Its tensile strength is ≥500MPa, yield strength is 250~310MPa, and elongation after fracture is over 30%, achieving high strength and excellent stamping performance for the outer panel steel.

Claims

1. A 500MPa grade cold-rolled high-strength steel for automotive outer panels based on rapid heat treatment technology, characterized in that, The chemical composition of the steel plate, by weight percentage, is as follows: C: 0.05%~0.11%; Mn: 0.6%~1.50%; Si: 0.10%~0.7%; P≤0.030%; N≤0.002%; S≤0.005%; one or more of Nb, Ti and V, and the content of Nb+Ti+V ≤0.02% ≤0.01%; the balance is Fe and unavoidable impurities.

2. The 500MPa grade cold-rolled high-strength steel for automotive outer panels based on rapid heat treatment technology according to claim 1, characterized in that, The microstructure of the steel plate includes: critical zone ferrite, oriented epiphytic ferrite, bainite, martensite, secondary martensite, and retained austenite, wherein each microstructure is expressed as a percentage by volume as follows: critical zone ferrite: 55%–70%, oriented epiphytic ferrite: 5%–15%, bainite + martensite: 8%–13%, secondary martensite: 5%–10%, and retained austenite: 5%–12%, and the grain size of the ferrite is 2–12 μm.

3. The 500MPa grade cold-rolled high-strength steel for automotive outer panels based on rapid heat treatment technology according to claim 1, characterized in that, The steel plate has a tensile strength ≥500MPa, a yield strength of 250~310MPa, and an elongation after fracture of 30%~36%.

4. A method for manufacturing 500MPa grade cold-rolled high-strength steel for automotive outer panels based on rapid heat treatment technology as described in any one of claims 1-3, characterized in that, This includes smelting, hot rolling, pickling, cold rolling, continuous annealing, and finishing. Hot-rolled: 1) Heating temperature 1200~1280℃, furnace time ≥120min; 2) Rough rolling temperature 1030~1100℃, final rolling temperature ≥880℃; 3) Intermediate cooling temperature 700~730℃, rear cooling rate >60℃ / s; 4) Winding temperature: 580–670℃; Cold rolling: The cold rolling reduction rate is controlled between 55% and 75%. Continuous annealing: 1) During the annealing process, the heating rate is 30-120℃ / s, the isothermal temperature is 760-850℃, and the annealing time is 5-20s; 2) The slow cooling rate is 2-8℃ / s, and the slow cooling temperature is 680-720℃; 3) After slow cooling, reduce the temperature to an aging temperature of 380–430°C at a rapid cooling rate of >18°C / s, with an aging time of 30–100s: 4) After the aging stage, cool to room temperature at a final cooling rate of >10℃ / s; Finishing: The elongation after finishing is between 0.2% and 0.8%.

5. A method for manufacturing 500MPa grade cold-rolled high-strength steel for automotive outer panels based on rapid heat treatment technology according to claim 4, characterized in that, The smelting process involves smelting in a converter and a refining furnace, followed by casting into billets.

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