Hot-dip aluminized silicon 980MPa grade Q&P steel cold-formed steel sheet and its preparation method

Abstract

The application provides a hot-dip aluminum-silicon 980MPa Q&P steel cold forming steel plate and a preparation method thereof, and the component of the hot-dip aluminum-silicon base plate is as follows in percentage by mass: 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 of Fe and inevitable impurities. The preparation method comprises a series of processes such as smelting, hot rolling, pickling, cold rolling, continuous annealing hot-dip aluminum-silicon, and finishing, and the hot-dip aluminum-silicon is ingeniously carried out in the slow cooling stage, perfectly matches the heat treatment system, breaks the limitation that the hot-dip galvanized steel plate must be over-aged at high temperature for a short time, and can make the bainite transformation sufficient; the hot-dip aluminum-silicon 980MPa 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 980MPa 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, coated Q&P steel is mainly achieved through hot-dip galvanizing of the steel plate surface. However, with the decreasing zinc resources and rising prices, there is an urgent need for a new hot-dip galvanizing method that matches the heat treatment process to meet the future market demand for Q&P steel with good surface quality and mechanical properties. Among them, aluminum-silicon coating is one of the coatings suitable for steel plates, which can give the steel plate better corrosion resistance than hot-dip galvanizing.

[0003] The relevant patent documents are as follows:

[0004] 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.

[0005] 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

[0006] The purpose of this invention is to provide a hot-dip aluminized silicon 980MPa grade Q&P steel cold-formed steel sheet with excellent mechanical properties and its preparation method.

[0007] The objective of this invention is achieved as follows:

[0008] A hot-dip aluminized silicon 980MPa grade Q&P steel cold-formed sheet, wherein the composition of the substrate of the cold-formed steel sheet by weight percentage is as follows: C: 0.18%~0.24%, Si: 1.4%~2.8%, Mn: 1.9%~2.5%, P≤0.02%, S≤0.005%, Ti: 0.01%~0.04%, Nb≤0.04%, Al≤0.005%, with the balance being Fe and unavoidable impurities.

[0009] The coating composition of the cold-formed steel sheet, by weight percentage, is as follows: Al: 88%–93%, Si: 7%–12%.

[0010] The final microstructure of the steel plate consists of ferrite, bainite, martensite, and retained austenite. The microstructure is calculated using the planar area method, with ferrite <40%, bainite 10%–60%, martensite 10%–60%, and retained austenite 6%–18%.

[0011] The steel plate has a tensile strength of ≥980MPa, a yield strength of 600-850MPa, and an elongation of ≥20%.

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

[0013] 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 microstructure, 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 980 MPa. Therefore, this application controls the carbon content at about 0.2%, with the optimal range being 0.18% to 0.24%.

[0014] 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 1.4%–2.8% in this invention.

[0015] 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 0.2%, in order to achieve a steel plate strength of 980 MPa, the Mn content is mainly controlled between 1.9% and 2.5%. Too low a Mn content will result in insufficient steel plate strength, while too high a content will reduce the plasticity of the steel plate.

[0016] 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%.

[0017] 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 Nb content to be less than or equal to 0.04%.

[0018] Al: Al has a strong deoxidizing ability and is used for deoxidation in converter steelmaking, making it an unavoidable element in steel. When w(Al) is greater than 0.003%, it may clog the nozzle due to the formation of Al2O3 inclusions.

[0019] 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%.

[0020] 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%.

[0021] The second technical solution of the present invention is to provide a method for preparing hot-dip aluminized silicon 980MPa grade Q&P steel cold-formed steel sheet, including smelting, hot rolling, pickling, cold rolling, continuous annealing aluminized silicon plating, and finishing.

[0022] Smelting: Smelting is carried out in a converter to obtain alloy compositions within the above range.

[0023] Hot-rolled:

[0024] ① The heating temperature is between 1220 and 1260℃ to ensure the precipitation behavior of Ti atoms, which has a good effect on solidifying N in the steel plate and ensures the precipitation of Ti (C, N), which plays a role in pinning the original austenite grain boundaries and refining the original austenite grains.

[0025] ② The initial rolling temperature is between 1100 and 1160℃, and the final rolling temperature is above 900℃ to ensure the rolling temperature of the recrystallization zone and promote the dynamic recrystallization behavior of the original austenite grains during the hot rolling stage.

[0026] ③ The coiling temperature should be between 560 and 700℃ to prevent excessively low coiling temperatures from increasing the difficulty of cold rolling. The thickness of the hot-rolled coil should be between 2.8 and 4.0 mm.

[0027] Pickling: Removes the iron oxide scale generated on the hot-rolled surface to ensure the surface quality of cold-rolled steel sheets.

[0028] 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.

[0029] Continuous annealing aluminum-silicon plating:

[0030] ① The heating isothermal temperature is 800-850℃, 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; these limited isothermal temperatures, times, and slow cooling rates can enable the steel plate to obtain <40% ferrite structure and original austenite phase.

[0031] ② Aluminized silicon plating: After slow cooling, the steel plate is immersed in an aluminum-silicon plating solution for aluminum-silicon plating. After slow cooling, the steel plate is immersed in the aluminum-silicon plating solution at 650-700℃ for plating. Preferably, the temperature of the aluminum-silicon plating solution is 650-700℃ and the plating time is 3-10s. The composition of the plating solution by weight percentage is as follows: Al: 88%-93%, Si: 7%-12%.

[0032] ③ After aluminizing with silicon, the steel plate is cooled to a quenching temperature of 220–360°C at a cooling rate greater than 10°C / s to obtain martensitic and supercooled austenitic structures. Then, the temperature is raised to an over-aging temperature of 360–420°C for an aging time of 200–650 s, followed by a cooling rate greater than 2°C / s to room temperature. This limited isothermal temperature and time allow C atoms in the martensite obtained in the previous stage to diffuse into the austenite, while the austenite undergoes a bainitic phase transformation to obtain 10%–60% bainite. A cooling rate greater than 2°C / s allows the steel plate to obtain martensitic structure and 6%–18% retained austenite.

[0033] The mechanism is as follows: First, annealing is performed in the austenitic or two-phase region to obtain <40% ferrite and original austenite; then, the temperature is slowly cooled to 650–700℃ for hot-dip aluminizing with silicon, followed by quenching and cooling to 220–360℃ to obtain martensitic structure. Subsequently, the temperature is raised to 360–420℃ isothermally, where carbon atoms in the martensite diffuse into the austenite, and bainitic phase transformation occurs simultaneously. Because the hot-dip aluminizing with silicon occurs during the slow cooling stage, the over-aging stage of this invention 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, enabling sufficient bainitic transformation, which is beneficial to the mechanical properties of the steel sheet.

[0034] Finishing: Then, the steel plate enters the finishing machine for shape adjustment, and the finishing elongation is controlled at 0.1% to 0.8%.

[0035] The final microstructure of the steel plate of this invention consists of ferrite, bainite, martensite, and retained austenite, with an aluminum-silicon coating on the surface. It is evident that this invention employs a novel continuous annealing aluminum-silicon coating process, resulting in Q&P steel with a tensile strength exceeding 980 MPa, a yield strength of 600–850 MPa, and an elongation greater than 20%, achieving a good combination of strength and plasticity in the steel plate.

[0036] The beneficial effects of this invention are as follows:

[0037] (1) The main chemical composition of the steel of the present invention is C and Mn, with no obvious precious alloys. At the same time, the C content is less than 0.24%, which makes the carbon equivalent of the steel plate low (carbon equivalent = C + Mn / 6 + (Cr + Mo + V) / 5 + (Ni + Cu) / 15). Therefore, the steel plate has excellent welding performance.

[0038] (2) This invention uses a unique continuous annealing aluminized silicon plating method to achieve high-performance Q&P steel. It cleverly performs hot-dip aluminized silicon plating during the slow cooling stage, which perfectly matches the heat treatment regime. 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.

[0039] (3) This invention achieves good strength and plasticity of steel plates through low-cost alloy design and ingenious process design. Attached Figure Description

[0040] Figure 1 This is a microstructure diagram of Embodiment 1 of the present invention. Detailed Implementation

[0041] The present invention will be further illustrated below through examples.

[0042] 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.

[0043] Hot rolling: heating temperature is 1220~1260℃, initial rolling temperature is 1100~1160℃, final rolling temperature is above 900℃, coiling temperature is 560~700℃, and hot-rolled coil thickness is 2.8~4.0mm.

[0044] Cold rolling: The cold rolling reduction rate is 50% to 58%;

[0045] Continuous annealing aluminum-silicon plating:

[0046] (1) Heating isothermal temperature 800~850℃, isothermal time 80~150s, slow cooling temperature 650~700℃, slow cooling rate 0.5~5℃ / s;

[0047] (2) Aluminized silicon plating: After slow cooling, the steel plate is immersed in aluminum-silicon liquid at 650-700℃ to form a coating; the composition of the plating solution by weight percentage is as follows: Al: 88%-93%, Si: 7%-12%;

[0048] (3) Then cool to the quenching temperature of 220-360℃ at a cooling rate of more than 10℃ / s, then raise the temperature to the over-aging temperature of 360-420℃, the over-aging time is 200-650s, and then cool to room temperature at a cooling rate of more than 2℃ / s.

[0049] Finishing: The finishing elongation is controlled between 0.1% and 0.8%.

[0050] Further; after step (1), the microstructure of the steel plate is ferrite and original austenite, with ferrite < 40% by volume percentage.

[0051] Further; in step (2) aluminum-silicon plating, the temperature of the aluminum-silicon plating solution is 650-700℃, and the plating time is 3-10s.

[0052] The composition of the steel in this embodiment of the invention is shown in Table 1. The main process parameters for continuous casting and hot rolling of the steel in this embodiment of the invention are shown in Table 2. The main process parameters for annealing and aluminizing / silicon plating 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.

[0053] Table 1. Composition (wt%) of steel in embodiments of the present invention

[0054] Example C Mn Si Ti Nb P S Al 1 0.19 2.32 2.36 0.025 0.015 0.010 0.005 0.002 2 0.18 2.46 1.85 0.02 0.02 0.009 0.005 0.003 3 0.20 2.23 1.48 0.015 0.03 0.010 0.003 0.001 4 0.20 2.52 1.80 0.02 0.02 0.005 0.005 0.003 5 0.21 2.18 1.93 0.03 - 0.009 0.003 0.004 6 0.23 1.91 2.62 0.015 0.03 0.008 0.005 0.002 7 0.19 2.2 2.1 0.04 - 0.01 0.005 0.005 8 0.22 2.1 2.2 0.015 0.02 0.02 0.004 0.001

[0055] Table 2 Main process parameters for continuous casting, hot rolling, and cold rolling of steel in the embodiments of the present invention.

[0056]

[0057] Table 3 Main process parameters for steel annealing and aluminizing / silicon plating in embodiments of the present invention.

[0058]

[0059] Table 4 Properties of steel in embodiments of the present invention

[0060] Example Rp0.2 / MPa Rm / MPa A50 / ％ 1 818 1011 21.1 2 786 1014 20.9 3 747 1017 22.3 4 721 1031 20.5 5 688 1008 20.7 6 697 1019 21.4 7 686 1022 20.6 8 655 1033 21.2

[0061] Table 5. Microstructure and plating solution composition (wt%) of the steel in the embodiments of the present invention.

[0062]

[0063] 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 hot-dip aluminized silicon 980MPa grade Q&P steel cold-formed sheet, characterized in that, The composition percentage of the substrate of the cold-formed steel plate is as follows: C: 0.18%~0.24%, Si: 2.1%~2.8%, Mn: 1.9%~2.2%, P≤0.02%, S≤0.005%, Ti: 0.01%~0.04%, Nb≤0.04%, Al≤0.005%, with the balance being Fe and unavoidable impurities; the microstructure of the steel plate is ferrite + bainite + martensite + retained austenite, wherein the microstructure is statistically analyzed by the planar area method, with ferrite 16%~40%, bainite 10%~60%, martensite 10%~60%, and retained austenite 6%~11%.

2. The hot-dip aluminized silicon 980MPa grade Q&P cold-formed steel sheet according to claim 1, characterized in that, The coating composition of cold-formed steel sheets by weight percentage is as follows: Al: 88%~93%, Si: 7%~12%.

3. The hot-dip aluminized silicon 980MPa grade Q&P cold-formed steel sheet according to claim 1, characterized in that, The steel plate has a tensile strength of ≥980MPa, a yield strength of 600~850MPa, and an elongation of ≥20%.

4. A method for preparing a hot-dip aluminized silicon 980MPa 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 plating, and finishing; characterized in that: Hot rolling: heating temperature 1220~1260℃, 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 aluminum-silicon plating: (1) Heating isothermal temperature 800~850℃, isothermal time 80~150s, slow cooling temperature 650~700℃, slow cooling rate 0.5~5℃ / s; (2) Aluminized silicon plating: After slow cooling, the steel plate is immersed in aluminum-silicon liquid at 650~700℃ to form a coating; the composition of the plating solution by weight percentage is as follows: Al: 88%~93%, Si: 7%~12%; (3) Then cool to the quenching temperature of 220~351℃ at a cooling rate of more than 25℃ / s, then raise the temperature to the over-aging temperature of 360~420℃, the over-aging time is 200~650s, and then cool down to room temperature at a cooling rate of more than 2℃ / s. Finishing: The finishing elongation rate is controlled at 0.1%~0.8%.

5. The hot-dip aluminized silicon 980MPa grade Q&P steel cold-formed steel sheet and its preparation method according to claim 4, characterized in that: After step (1), the microstructure of the steel plate consists of ferrite and original austenite, with ferrite accounting for less than 40% by volume percentage.

6. The hot-dip aluminized silicon 980MPa grade Q&P steel cold-formed steel sheet and its preparation method according to claim 4, characterized in that: In step (2) aluminum-silicon plating, the temperature of the aluminum-silicon plating solution is 650~700 ℃, and the plating time is 3~10s.

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