Hot-dip aluminized silicon 1180MPa grade Q&P steel cold-formed steel sheet and its preparation method

Abstract

The application provides a hot-dip aluminized silicon 1180MPa grade Q&P steel and a preparation method thereof, and the base plate component of the hot-dip aluminized silicon is as follows in terms of mass percentage: C: 0.18% to 0.24%, Si: 1.4% to 2.8%, Mn: 1.9% to 2.5%, P: less than or equal to 0.02%, S: less than or equal to 0.005%, Ti: 0.01 to 0.04%, Nb: less than or equal to 0.04%, Al: less than or equal to 0.005%, and the balance is Fe and inevitable impurities. The preparation method comprises a series of processes such as smelting, hot rolling, pickling, cold rolling, continuous annealing hot-dip aluminized silicon and finishing, and the hot-dip aluminized silicon 1180MPa grade Q&P steel produced by the application has excellent strength and plasticity.

Description

Technical Field

[0001] This invention belongs to the field of metallic materials, and particularly relates to a hot-dip aluminized silicon 1180MPa grade Q&P steel cold-formed steel sheet and its preparation method. Background Technology

[0002] With increasingly stringent requirements from the automotive industry regarding lightweighting and safety, the proportion of advanced high-strength steel used in new car models continues to rise. In the body structure designed for the "Ultra-Light Steel Body—Advanced Car Concept" project, high-strength steel with a tensile strength of 1000MPa accounts for the largest proportion, approximately 29% to 30% of the car body weight. High-strength steels with a tensile strength above 1000MPa mainly include dual-phase steel, martensitic steel, multiphase steel, and Q&P steel. Among them, Q&P steel belongs to the third generation of advanced high-strength steel, possessing high strength and high plasticity, capable of meeting the stamping forming requirements of complex automotive parts. Q&P steel has high energy absorption capacity, good mechanical properties, formability, and weldability, and is widely used in the production of automotive chassis suspension components, B-pillars, bumpers, and other parts, showing broad market prospects. Currently, the strength of Q&P steel produced in the international industry can reach over 1000MPa, with the highest reaching 1500MPa. Among them, Q&P high-strength steel plates with coatings are usually achieved by hot-dip galvanizing. However, with the decreasing zinc resources and rising prices, there is an urgent need for a new hot-dip galvanizing method to meet the future market demand for coated high-strength Q&P steel.

[0003] The relevant literature is as follows: Chinese patent application CN104278194A discloses a high-strength, high-ductility quenched steel plate and its preparation method. It employs a high-C, high-Si / Al composition system. Besides its strengthening effect, C is used to stabilize retained austenite; Si / Al elements are used to inhibit carbide precipitation, indirectly stabilizing retained austenite. Specifically, the C content is 0.25–0.35 wt%, the Si content is 0.8–1.2 wt%, and the Al content is 0.5–1.0 wt%. The steel plate obtained after heat treatment has a tensile strength greater than 980 MPa and an elongation of approximately 20%, without involving surface hot-dip galvanizing.

[0004] Chinese patent application CN113061812 A discloses a 980MPa grade cold-rolled alloyed galvanized quenched steel and its manufacturing method. The steel composition has a low carbon content (0.1%–0.17%), the annealing temperature is in the fully austenitic region, and the steel plate surface undergoes hot-dip galvanizing and alloying, without aluminum-silicon coating. After heat treatment, the steel plate yields a tensile strength ≥980MPa, a yield strength ≥800MPa, a hole expansion rate ≥60%, and an elongation ≥12%. Summary of the Invention

[0005] The purpose of this invention is to provide a hot-dip aluminized silicon 1180MPa grade Q&P cold-formed steel sheet with excellent mechanical properties and its preparation method.

[0006] The objective of this invention is achieved as follows:

[0007] A hot-dip aluminized silicon 1180MPa grade Q&P steel cold-formed sheet, the composition of the substrate of the cold-formed steel sheet by weight percentage is as follows: C: 0.18%~0.25%, Si: 1.0%~2.0%, Mn: 2.2%~3.0%, Ti: 0.01%~0.04%, Nb≤0.04%, Al≤0.005%, P≤0.02%, S≤0.005%, with the balance being Fe and unavoidable impurities;

[0008] The composition of the coating on the cold-formed steel sheet, by weight percentage, is as follows: Al content 88%–93%, Si content 7%–12%.

[0009] The microstructure of the steel plate consists of ferrite, bainite, martensite, and retained austenite. The microstructure is statistically analyzed using the planar area method, with ferrite ≤30%, bainite 10%–60%, martensite 10%–60%, and retained austenite 6%–18%.

[0010] The steel plate has a tensile strength of ≥1180MPa, a yield strength of 800~1050MPa, and an elongation of ≥14%.

[0011] The rationale for the steel plate composition design of this invention is as follows:

[0012] C: Carbon is a traditional and economical strengthening element for low-carbon steel. However, excessive carbon content can inhibit bainitic phase transformation, increase the hardness difference between phases in the steel plate structure, and reduce the expansion performance of the steel plate. It can also cause difficulties in smelting and welding. If the carbon content is too low, it is difficult to make the steel plate reach 1180 MPa. Therefore, this application controls the carbon content to 0.18% to 0.25%.

[0013] Si: Si strengthens ferrite. Furthermore, Si primarily inhibits cementite precipitation during the over-aging stage, which helps retain residual austenite and improves the plasticity of the steel plate. Therefore, the Si content is controlled at 1.0%–2.0% in this invention.

[0014] Mn: Mn is an austenite stabilizing element with a significant solid solution strengthening effect on high-strength steel. It can significantly improve the hardenability of steel, and has the functions of solid solution strengthening and refining ferrite grains. It can significantly delay the transformation of pearlite and bainite, thereby increasing the strength of steel. It is the main strengthening element besides carbon. Since the carbon content is fixed at about 0.22%, in order to achieve a steel plate strength of 1180 MPa, the Mn content is mainly controlled between 2.2% and 3.0%. Too low an Mn content may result in insufficient steel plate strength, while too high an Mn content will reduce the plasticity of the steel plate.

[0015] Ti: Ti can capture free nitrogen atoms in steel, thus consolidating nitrogen. Simultaneously, TiN can precipitate during solidification, pinning grain boundaries. Ti(C,N) precipitation during hot rolling pins the original austenite grain boundaries, refining the original austenite grains. A small amount of Ti precipitates during continuous annealing, strengthening ferrite and bainite; however, adding too much Ti has limited effect and increases cost. Therefore, in this invention, the Ti content is controlled at 0.01%–0.04%.

[0016] Nb plays a significant role in grain refinement, phase transformation behavior, C enrichment in austenite, and martensite nucleation. Nb combines with C and N to form fine carbonitrides, inhibiting grain growth and resulting in a significant strengthening effect. Therefore, this application controls the Mn content to ≤0.04%.

[0017] Al: Al is produced during the steelmaking process and is usually used as a strong deoxidizer to deoxidize molten steel. Therefore, it is an impurity element in this invention, and the Al content is controlled at ≤0.005%.

[0018] P: P is a harmful element in steel, and its content should be as low as possible. Considering cost, the P content in this invention is controlled at P≤0.02%.

[0019] S: S is a harmful element in steel, and its content should be as low as possible. Considering cost, the S content in this invention is controlled to S≤0.005%.

[0020] The second technical solution of the present invention is to provide a method for preparing hot-dip aluminized silicon 1180MPa grade Q&P steel cold-formed steel sheet, including smelting, hot rolling, pickling, cold rolling, continuous annealing aluminized silicon coating, and finishing.

[0021] Smelting: Smelting is carried out in a converter to obtain alloy compositions within the above range.

[0022] Hot rolling: heating temperature 1220~1260℃, holding time ≥100min, initial rolling temperature 1100~1160℃, final rolling temperature above 900℃, coiling temperature 560~700℃, hot rolled coil thickness 2.8~4.0mm;

[0023] The heating temperature is between 1220 and 1260℃, with a holding time ≥100 min, to ensure Ti atom precipitation, effectively consolidating N in the steel plate and guaranteeing Ti(C, N) precipitation. This helps to pin the original austenite grain boundaries and refine the original austenite grains. The initial rolling temperature is between 1100 and 1160℃, and the final rolling temperature is above 900℃ to ensure the rolling temperature in the recrystallization zone, promoting the dynamic recrystallization of the original austenite grains during hot rolling. The coiling temperature is between 560 and 700℃ to prevent excessively low coiling temperatures from increasing the difficulty of cold rolling. Simultaneously, Nb C and N compounds precipitate within this temperature range, achieving precipitation strengthening. The thickness of the hot-rolled coil is between 2.8 and 4.0 mm.

[0024] Pickling: Removes the iron oxide scale generated on the hot-rolled surface to ensure the surface quality of cold-rolled steel sheets;

[0025] Cold rolling: The cold rolling reduction rate is 50% to 58%, ensuring that the cold rolling reduction is more than 50% to promote the fiberization of the microstructure in the cold rolling configuration; at the same time, it prevents the cold rolling reduction rate from being too high, which would lead to excessive deformation resistance and make it difficult to roll to the target thickness.

[0026] Continuous annealing:

[0027] ① The heating isothermal temperature is 820-870℃, the isothermal time is 80-150s, the slow cooling temperature is 650-700℃, and the slow cooling rate is controlled at 0.5-5℃ / s. This heating temperature and isothermal time allow the cold-rolled deformation structure of the steel plate to recover and recrystallize in the austenite region or the austenite and ferrite dual-phase region. Slow cooling to 650-700℃ and controlling the slow cooling rate at 0.5-5℃ / s can transform the austenite phase in the steel plate into ferrite, thereby adjusting the final microstructure of the steel plate.

[0028] ② Aluminized silicon plating: After slow cooling, immerse in aluminum-silicon plating solution for aluminum-silicon plating. The temperature of the aluminum-silicon plating solution is 650-700℃, and the plating time is 3-10s. Limiting the temperature range for aluminum-silicon plating can improve the fluidity of the aluminum-silicon plating solution and ensure the smooth progress of the coating, so that the elements in the steel plate can be further diffused evenly, which helps to improve the uniformity of the steel plate structure.

[0029] ③ After aluminizing with silicon, the steel is cooled to a quenching temperature of 240–340°C at a rate greater than 10°C / s, then heated to an over-aging temperature of 360–420°C for 200–650s, and then cooled to room temperature at a rate greater than 2°C / s. Cooling to 240–340°C at a rate greater than 10°C / s ensures that some austenite in the steel plate undergoes martensitic transformation. The isothermal temperature of 360–420°C and the isothermal time of 200–650s allow the martensitic phase to temper and allows the carbon element inside to diffuse into the adjacent austenite to stabilize the austenite phase. At the same time, this stage ensures that some austenite undergoes sufficient bainitic transformation to obtain a 10%–60% bainitic structure, while allowing the carbon element in the bainite to diffuse into its adjacent austenite phase to stabilize it. Cooling rates greater than 2℃ / s cause some of the austenite in the steel to undergo martensitic transformation, enhancing the strength of the steel plate. At the same time, the steel plate ultimately obtains 6% to 18% of retained austenite, improving the strength and plasticity of the steel plate.

[0030] Finishing: The steel sheet enters the finishing machine for shape adjustment, and the finishing elongation is controlled between 0.1% and 0.6%.

[0031] The mechanism is as follows: First, annealing is performed in the austenitic or two-phase region to form a multiphase structure of ferrite and original austenite; then, hot-dip aluminizing and siliconizing is carried out at 650–700℃, followed by isothermal cooling to 240–340℃ to obtain a certain amount of martensite; subsequently, the temperature is raised to 360–420℃, where carbon atoms in the martensite diffuse into the austenite, and bainitic phase transformation occurs simultaneously. Because the hot-dip aluminizing and siliconizing occurs during the slow cooling stage, the over-aging stage time can be flexibly adjusted according to the phase transformation requirements, breaking the limitation that hot-dip galvanized steel sheets must undergo high-temperature, short-time over-aging. This allows for sufficient bainitic transformation, which is beneficial to the mechanical properties of the steel sheet.

[0032] The final microstructure of the steel plate of this invention consists of ferrite + bainite + martensite + retained austenite, and the surface of the steel plate is coated with an aluminum-silicon coating. It is evident that this invention employs a novel hot-dip galvanizing process combined with a heat treatment process, resulting in Q&P steel with a tensile strength exceeding 1180 MPa, a yield strength of 800–1050 MPa, and an elongation ≥14%, achieving excellent strength and plasticity in the hot-dip aluminum-silicon coated steel plate.

[0033] The beneficial effects of this invention are as follows:

[0034] (1) The chemical composition of the steel of the present invention is mainly composed of C and Mn as the main elements, with no obvious precious alloys. At the same time, the C content is less than 0.25%, which is beneficial to laser welding and resistance spot welding in the production and application process.

[0035] (2) The present invention uses a novel hot-dip aluminum-silicon method to achieve a surface coating on Q&P steel, and cleverly performs hot-dip aluminum-silicon coating during the slow cooling stage, which perfectly matches the heat treatment system. This breaks the limitation that hot-dip galvanized steel sheets must be subjected to high temperature and short-time aging, which can make the bainite transformation sufficient and is beneficial to the mechanical properties of the steel sheet.

[0036] (3) This invention achieves good strength and plasticity of steel plates through low-cost alloy design and ingenious process design. Attached Figure Description

[0037] Figure 1 This is a metallographic image of the microstructure of Embodiment 1 of the present invention. Detailed Implementation

[0038] The present invention will be further illustrated below through examples.

[0039] According to the component ratio of the technical solution, the embodiments of the present invention carry out smelting, hot rolling, pickling, cold rolling, continuous annealing, aluminizing and silicon plating, and finishing.

[0040] Hot rolling: heating temperature 1220~1260℃, holding time ≥100min, initial rolling temperature 1100~1160℃, final rolling temperature above 900℃, coiling temperature 560~700℃, hot rolled coil thickness 2.8~4.0mm;

[0041] Cold rolling: The cold rolling reduction rate is 50% to 58%;

[0042] Continuous annealing of aluminum-silicon plating:

[0043] ① The heating isothermal temperature is 820~870℃, the isothermal time is 80~150s, the slow cooling temperature is 650~700℃, and the slow cooling rate is controlled at 0.5~5℃ / s;

[0044] ② Aluminized silicon plating: After slow cooling, immerse in aluminum-silicon plating solution for aluminum-silicon plating. The temperature of the aluminum-silicon plating solution is 650-700℃, and the plating time is 3-10 seconds.

[0045] ③ After aluminum-silicon plating, cool to the quenching temperature of 240-340℃ at a cooling rate of more than 10℃ / s, then raise the temperature to the over-aging temperature of 360-420℃ and wait for 200-650s, then cool to room temperature at a cooling rate of more than 2℃ / s.

[0046] Finishing: The finishing elongation is controlled between 0.1% and 0.6%.

[0047] The composition of the steel in this embodiment of the invention is shown in Table 1. The main process parameters for continuous casting, hot rolling, and cold rolling of the steel in this embodiment of the invention are shown in Table 2. The main process parameters for annealing and aluminizing / silicon coating of the steel in this embodiment of the invention are shown in Table 3. The properties of the steel in this embodiment of the invention are shown in Table 4.

[0048] Table 1. Composition (wt%) of steel in embodiments of the present invention

[0049] Example C Mn Si Ti Nb Al P S 1 0.19 2.9 1.3 0.025 0.025 0.002 0.010 0.005 2 0.20 2.4 1.2 0.032 0.03 0.001 0.009 0.005 3 0.22 2.2 1.1 0.015 0.02 0.003 0.010 0.003 4 0.23 2.5 1.5 0.021 0.02 0.002 0.005 0.005 5 0.21 2.3 1.4 0.023 - 0.004 0.009 0.003 6 0.23 2.5 1.7 0.025 0.02 0.001 0.008 0.005 7 0.24 2.4 1.4 0.037 - 0.005 0.01 0.005 8 0.22 2.6 1.8 0.014 0.03 0.003 0.02 0.004

[0050] Table 2 Main process parameters for hot rolling and cold rolling of steel in the embodiments of the present invention.

[0051]

[0052] Table 3 Main process parameters for aluminum-silicon stripping of steel in embodiments of the present invention.

[0053]

[0054] Table 4 Properties of steel in embodiments of the present invention

[0055] Example Rp0.2 / MPa Rm / MPa A50 / ％ 1 1021 1211 16.1 2 987 1214 14.9 3 943 1217 15.4 4 911 1231 15.8 5 887 1198 14.7 6 873 1189 16.3 7 846 1222 15.6 8 825 1193 14.7

[0056] Table 5. Microstructure and plating solution composition (wt%) of the steel in the embodiments of the present invention.

[0057]

[0058] The steel plates produced using this invention have a tensile strength of over 1180 MPa, a yield strength of 800–1050 MPa, and an elongation of ≥14%. The microstructure consists of ferrite, bainite, martensite, fresh martensite, and retained austenite. Statistically, based on the planar area method, the microstructure comprises ≤30% ferrite, 10%–60% bainite, 10%–60% martensite, and 6%–18% retained austenite.

[0059] To illustrate the present invention, the present invention has been appropriately and sufficiently described above through embodiments. The above embodiments are only for illustrating the present invention and are not intended to limit the present invention. Those skilled in the art can make various changes and modifications without departing from the spirit and scope of the present invention. Any modifications, equivalent substitutions, improvements, etc., should be included within the protection scope of the present invention. The patent protection scope of the present invention should be defined by the claims.

Claims

1. A cold-formed steel sheet of hot-dip aluminized silicon 1180MPa grade Q&P steel, characterized in that, The composition of the cold-formed steel plate substrate, by weight percentage, is as follows: C: 0.18%~0.25%, Si: 1.0%~1.7%, Mn: 2.3%~3.0%, Ti: 0.01%~0.04%, Nb≤0.04%, Al≤0.005%, P≤0.02%, S≤0.005%, with the balance being Fe and unavoidable impurities. The microstructure of the steel plate consists of ferrite, bainite, martensite, and retained austenite. The microstructure, calculated by the planar area method, is as follows: ferrite ≤15%, bainite 10%~60%, martensite 10%~60%, and retained austenite 12%~18%.

2. The hot-dip aluminized silicon 1180MPa grade Q&P cold-formed steel sheet according to claim 1, characterized in that, The composition of the coating on the cold-formed steel sheet by weight percentage is as follows: Al: 88%~93%, Si: 7%~12%.

3. The hot-dip aluminized silicon 1180MPa grade Q&P cold-formed steel sheet according to claim 1, characterized in that, The steel plate has a tensile strength of ≥1180MPa, a yield strength of 800~1050MPa, and an elongation of ≥14%.

4. A method for preparing a hot-dip aluminized silicon 1180MPa grade Q&P steel cold-formed sheet according to any one of claims 1-3, comprising smelting, hot rolling, pickling, cold rolling, continuous annealing with aluminized silicon, and finishing; characterized in that: Hot rolling: heating temperature 1220~1260℃, holding time ≥100 min, initial rolling temperature 1100~1160℃, final rolling temperature above 900℃, coiling temperature 560~700℃, hot rolled coil thickness 2.8~4.0mm; Cold rolling: The cold rolling reduction rate is 50%~58%; Continuous annealing of aluminum-silicon plating: ① Heating isothermal temperature 820~870℃, isothermal time 80~150s, slow cooling temperature 650~700℃, slow cooling rate controlled 0.5~5℃ / s; ② Aluminum-silicon plating: After slow cooling, immerse in aluminum-silicon plating solution for aluminum-silicon plating. The temperature of the aluminum-silicon plating solution is 650~700℃, and the plating time is 3~10s. ③ After aluminum-silicon plating, cool to the quenching temperature of 240~340℃ at a cooling rate of greater than 10℃ / s and less than or equal to 23℃ / s, then raise the temperature to the over-aging temperature of 360~420℃ and hold for 200~650s, then cool to room temperature at a cooling rate of greater than 2℃ / s. Finishing: The finishing elongation rate is controlled at 0.1%~0.6%.

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