Hot-dip aluminized silicon 1470MPa grade Q&P steel cold-formed steel sheet and its preparation method

Abstract

The application provides a hot-dip aluminized silicon 1470MPa grade Q&P steel cold forming steel plate, and the main components of the base plate of the hot-dip aluminized silicon are as follows in percentage by mass: C: 0.18% to 0.28%, Si: 1.0% to 2.0%, Mn: 2.4% to 3.2%, P: 0.02% or less, S: 0.005% or less, Ti: 0.01% to 0.04%, B: 0.002% to 0.005%, and one or more of Cr, Mo, Cu and Nb can be additionally added, wherein Cr: 0.1% to 0.4%, Mo: 0.1% to 0.4%, Cu: 0.2% to 0.6%, Nb: 0.01% to 0.04%, and the balance is Fe and inevitable impurities. The hot-dip aluminized silicon is ingeniously carried out in the slow cooling stage, is perfectly matched with 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, can make the bainite transformation sufficient, and is beneficial to the mechanical properties of the steel plate; the hot-dip aluminized silicon 1470MPa grade Q&P steel produced by the application has super-high strength and good plasticity at the same time.

Description

Technical Field

[0001] This invention belongs to the field of metallic materials, and particularly relates to a hot-dip aluminized silicon 1470MPa grade Q&P steel cold-formed steel sheet and its preparation method. Background Technology

[0002] With increasingly stringent requirements from the automotive industry for 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 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 industrially produced Q&P steel internationally can reach over 1000MPa, with some reaching up to 1500MPa, but only a small portion of Q&P steel sheets meet high surface quality requirements. Currently, high surface quality Q&P steel is mainly achieved through hot-dip galvanizing of steel plates. 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.

[0003] The relevant literature is as follows:

[0004] Chinese patent application CN106244918B discloses a 1500MPa high-strength-ductility-product automotive steel and its manufacturing method. It employs a high-Mn composition system, with an Mn content ranging from 7.5% to 12%, and also contains elements such as carbon, silicon, and aluminum, with C: 0.1% to 0.3%, Si: 0.1% to 2.0%, and Al: 0.01% to 2.0%. The production process utilizes a double-annealing + tempering heat treatment process and does not involve hot-dip coating. The resulting steel sheet has a tensile strength greater than 1500MPa and a strength-ductility-product of not less than 30GPa.

[0005] Chinese patent application CN114012056A discloses a 1500MPa grade hot-formed steel and its preparation method. The steel's composition system consists of carbon, silicon, manganese, chromium, titanium, and boron. Specifically, the composition ranges are: C: 0.19–0.26%, Si: 0.05–1.3%, Mn: 0.9–2.1%, B: 0.002–0.020%, Cr: 0.15–2.0%, and Ti: 0.02–0.15%. The production process employs hot forming and does not involve surface coating. Summary of the Invention

[0006] The purpose of this invention is to overcome the above-mentioned problems and deficiencies and provide a hot-dip aluminized silicon 1470MPa grade Q&P steel cold-formed steel sheet and its preparation method.

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

[0008] A hot-dip aluminized silicon 1470MPa grade Q&P steel cold-formed sheet, the composition of the substrate of the steel sheet by weight percentage is as follows: C: 0.18%~0.28%, Si: 1.0%~2.0%, Mn: 2.4%~3.2%, P≤0.02%, S≤0.005%, Ti: 0.01%~0.04%, B: 0.002%~0.005%; the balance is Fe and unavoidable impurities.

[0009] The cold-formed steel plate substrate further includes one or more of Cr, Mo, Cu, and Nb, wherein Cr: 0.1% to 0.4%, Mo: 0.1% to 0.4%, Cu: 0.2% to 0.6%, and Nb: 0.01% to 0.04%.

[0010] 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%.

[0011] The microstructure of the cold-formed steel sheet consists of ferrite, bainite, tempered martensite, fresh martensite, and retained austenite; wherein, based on the planar area method, the microstructure is ≤20% ferrite, 10%–60% bainite, 20%–45% tempered martensite, 10%–20% fresh martensite, and 6%–18% retained austenite.

[0012] The cold-formed steel sheet has a tensile strength of ≥1470MPa, a yield strength of 950-1300MPa, and an elongation of ≥8%.

[0013] The rationale for the design of the components in this invention is as follows:

[0014] 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 lead to a decrease in the expansion and plasticity 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 1470 MPa. Therefore, this application controls the carbon content at about 0.24%, with the optimal range being 0.18% to 0.28%.

[0015] 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.

[0016] 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 effects 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, but its content is too high, which will reduce the plasticity of steel plates. In order to achieve a steel plate strength of over 1470 MPa, the carbon content in the steel is required to be 0.18% to 0.28%, therefore the Mn content is controlled between 2.4% and 3.2%.

[0017] Cu: Cu plays a role in solid solution strengthening, replacing some of the Mn element and reducing Mn segregation. Furthermore, through proper process control, Cu can precipitate to form a Cu-rich phase, resulting in precipitation strengthening. Therefore, the Cu content in steel is 0.2%–0.6%.

[0018] 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%.

[0019] B: B element can effectively inhibit the formation of ferrite during slow cooling and coating stages, ensuring sufficient strength of the steel plate. However, excessive B will impair the plasticity of the steel plate. Therefore, the B element content is controlled at 0.002% to 0.005%.

[0020] 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 between 0.01% and 0.04%.

[0021] Cr: Cr can significantly stabilize the austenite phase region and delay the transformation of pearlite and bainite, effectively improving the hardenability of austenite. If its content is too low, it will affect the hardenability of the steel plate, and if it is too high, it will increase the cost. Therefore, this invention requires the Cr content to be controlled at 0.1-0.4%.

[0022] Mo: Mo is a strengthening element for steel and can also effectively improve the hardenability of steel plates. However, excessive addition of Mo will significantly increase the cost of steel. Therefore, this invention requires the Mo content to be controlled at 0.1-0.4%.

[0023] 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%.

[0024] 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%.

[0025] The second technical solution of the present invention is to provide a method for preparing hot-dip aluminized silicon 1470MPa grade Q&P steel cold-formed steel sheet, including smelting, hot rolling, pickling, cold rolling, continuous annealing aluminized silicon plating, and finishing.

[0026] 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 between 2.8~4.0mm.

[0027] ① The heating temperature is 1220–1260℃ to ensure the precipitation of Ti atoms, which effectively solidifies N in the steel plate and ensures the precipitation of Ti(C,N), thus pinning the original austenite grain boundaries and refining the original austenite grains. ② The initial rolling temperature is 1100–1160℃, and the final rolling temperature is above 900℃ to ensure the rolling temperature in the recrystallization zone and promote the dynamic recrystallization of the original austenite grains during the hot rolling stage. ③ The coiling temperature is 560–700℃ to prevent excessively low coiling temperatures from increasing the difficulty of cold rolling. The thickness of the hot-rolled coil is 2.8–4.0 mm.

[0028] Pickling: Removes the iron oxide scale generated on the hot-rolled surface to ensure the surface quality of cold-rolled steel sheets.

[0029] Cold rolling: The cold rolling reduction rate is 50% to 58%, ensuring a rolling reduction of more than 50% to promote the fiberization of the microstructure in the cold rolling configuration; at the same time, an excessively high cold rolling reduction rate will lead to excessive deformation resistance, making it difficult to roll to the target thickness.

[0030] Continuous annealing aluminum-silicon plating:

[0031] ① Annealing: The cold-rolled steel sheet is heated to an isothermal temperature of 860-920℃ and the isothermal time is 80-150s. The isothermal temperature and isothermal time within this range can cause the cold-rolled structure of the steel sheet to recrystallize and obtain ferrite and austenite phases or all austenite phases, laying the foundation for the transformation of the final microstructure of the steel sheet.

[0032] ② Slow cooling: After annealing and isothermal treatment, the steel plate is subjected to slow cooling treatment. The slow cooling temperature is 650~700℃, and the slow cooling rate is controlled at 0.5~5℃ / s. During the slow cooling stage, the steel plate undergoes ferrite phase transformation, and at the same time, the C element in the ferrite diffuses into the austenite phase, thereby stabilizing the austenite.

[0033] ③ Hot-dip aluminizing and siliconizing: After slow cooling, the steel plate is placed in an aluminum bath for hot-dip galvanizing. The temperature of the aluminum-silicon plating bath is 650–700℃, and the plating time is 3–10 seconds. This limited temperature range ensures the fluidity of the plating bath, guarantees the coating, and allows for further diffusion of elements within the ferrite and austenite phase regions of the steel plate, achieving a uniform element distribution. Preferably, the composition of the plating bath in the aluminum bath, by weight percentage, is as follows: Al content 88%–93%, Si content 7%–12%.

[0034] ④ After hot-dip aluminized silicon, cool to a quenching temperature of 320-360℃ at a quenching rate greater than 10℃ / s, then raise the temperature to an over-aging temperature of 380-460℃ for an over-aging time of 30-650s, and then cool to room temperature at a cooling rate greater than 2℃ / s. During this process, a quenching rate greater than 10℃ / s and a quenching temperature within this range can effectively prevent the formation of pearlite and other transformations, ensuring that the austenite in the steel plate undergoes a martensitic phase transformation to obtain 20%-45% martensite. The subsequent over-aging stage causes the quenched martensite to undergo a tempering reaction to form tempered martensite, while ensuring that the austenite undergoes a full bainitic transformation. At the same time, the carbon element in the tempered martensite and bainite diffuses fully into the remaining austenite phase to further heat the austenite phase. Finally, a cooling rate of more than 2℃ / s causes the metastable austenite in the steel plate to undergo a martensitic transformation to form 10-20% fresh martensite, while the stable austenite is retained as the residual austenite in the steel plate, which is 6%-18%.

[0035] Finishing: Finally, the steel plate enters the finishing machine for shape adjustment, and the finishing elongation is controlled at 0.1% to 0.6%.

[0036] The mechanism is as follows: First, annealing is performed in the austenitic or two-phase region to obtain a suitable proportion of ferrite and original austenite; then, the temperature is slowly cooled to 650–700℃ for hot-dip aluminizing with silicon, followed by isothermal cooling to 320–360℃ to obtain a certain amount of martensite; subsequently, the temperature is raised to 380–460℃ for isothermal treatment, during which 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. This allows for sufficient bainitic transformation, which is beneficial to the mechanical properties of the steel sheet.

[0037] The beneficial effects of this invention are as follows:

[0038] (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 0.28%, which is beneficial to laser welding and resistance spot welding in the production and application process.

[0039] (2) The present invention uses a novel hot-dip aluminum-silicon coating method to obtain an aluminum-silicon 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.

[0040] (3) This invention achieves both high strength and good plasticity in steel plates through low-cost alloy design and ingenious process design. Attached Figure Description

[0041] Figure 1 This is a metallographic image of the microstructure of Example 1. Detailed Implementation

[0042] The present invention will be further illustrated below through examples.

[0043] 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.

[0044] 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 between 2.8~4.0mm.

[0045] Cold rolling: The cold rolling reduction rate is 50% to 58%;

[0046] Continuous annealing aluminum-silicon plating:

[0047] ① Annealing: Heat the cold-rolled steel sheet to an isothermal temperature of 860–920℃, and the isothermal time is 80–150 seconds.

[0048] ② Slow cooling: After annealing and isothermal treatment, the steel plate is subjected to slow cooling treatment. The slow cooling temperature is 650-700℃, and the slow cooling rate is controlled at 0.5-5℃ / s.

[0049] ③ Hot-dip aluminum-silicon plating: After slow cooling, the steel plate is put into an aluminum pot for hot-dip plating treatment. The temperature of the aluminum-silicon plating solution is 650-700℃, and the plating time is 3-10s. Preferably, the composition of the plating solution in the aluminum pot is as follows by weight percentage: Al content 88%-93%, Si content 7%-12%.

[0050] ④ After hot-dip aluminized silicon, cool to the quenching temperature of 320-360℃ at a cooling rate of more than 10℃ / s, then raise the temperature to the over-aging temperature of 380-460℃, the over-aging time is 30-650s, and then cool down to room temperature at a cooling rate of more than 2℃ / s.

[0051] Finishing: The finishing elongation is controlled between 0.1% and 0.6%.

[0052] The composition of the steel in this embodiment of the invention is shown in Table 1. The main process parameters for hot rolling and cold rolling of the steel in this embodiment of the invention are shown in Table 2. The main process parameters for continuous 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] No. C Mn Cr Mo Cu Si Ti Nb B P S 1 0.19 2.9 0.3 0.2 - 1.3 0.025 0.025 0.002 0.010 0.005 2 0.20 2.4 0.2 - - 1.2 0.03 0.03 0.003 0.009 0.005 3 0.22 2.6 0.1 0.1 - 1.1 0.015 0.02 0.002 0.010 0.003 4 0.23 2.5 - - 0.2 1.5 0.02 0.02 0.004 0.005 0.005 5 0.21 2.6 - - - 1.4 0.02 - 0.003 0.009 0.003 6 0.23 2.5 0.2 - 0.4 1.7 0.025 0.02 0.002 0.008 0.005 7 0.24 2.5 0.3 - - 1.4 0.03 - 0.004 0.01 0.005 8 0.22 2.6 - 0.2 0.6 1.8 0.015 0.03 0.003 0.02 0.004

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

[0056]

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

[0058]

[0059] Table 4. Properties of the steel and composition of the aluminum-silicon plating solution (wt%) of the embodiments of the present invention.

[0060]

[0061] Table 5. Microstructure proportions of steel in the embodiments of the present invention

[0062]

[0063] As can be seen from the above embodiments, through low-cost alloy design and ingenious process design, hot-dip aluminized silicon steel plates with tensile strength of over 1470MPa, yield strength of 1000-1350MPa, and elongation of greater than or equal to 8% can be prepared, achieving ultra-high strength and good plasticity of the steel plate.

[0064] 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 1470MPa grade Q&P steel, characterized in that, The composition of the cold-formed steel sheet substrate, by weight percentage, is as follows: C: 0.18%–0.28%, Si: 1.1%–2.0%, Mn: 2.4%–3.2%, P≤0.02%, S≤0.005%, Ti: 0.01%–0.04%, B: 0.002%–0.005%, with the balance being Fe and unavoidable impurities. The microstructure of the cold-formed steel sheet consists of ferrite, bainite, tempered martensite, fresh martensite, and retained austenite. The microstructure, calculated by planar area method, is as follows: ferrite ≤20%, bainite 10%–60%, tempered martensite 20%–45%, fresh martensite 10%–20%, and retained austenite 6%–18%. The cold-formed steel sheet has a tensile strength of ≥1470 MPa, a yield strength of 950–1300 MPa, and an elongation ≥8%.

2. The hot-dip aluminized silicon 1470MPa grade Q&P cold-formed steel sheet according to claim 1, characterized in that, The cold-formed steel plate substrate also includes one or more of Cr, Mo, Cu, and Nb. The composition is as follows: Cr: 0.1%–0.4%, Mo: 0.1%–0.4%, Cu: 0.2%–0.6%, Nb: 0.01%–0.04%.

3. The hot-dip aluminized silicon 1470MPa 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 content 88%–93%, Si content 7%–12%.

4. A method for preparing a hot-dip aluminized silicon 1470MPa 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: Cold rolling reduction rate 50%–58%; Continuous annealing aluminum-silicon plating: ① Annealing: Heat the cold-rolled steel sheet to an isothermal temperature of 860-920℃ for 80-150 seconds; ② Slow cooling: After annealing and isothermal treatment, the steel plate is subjected to slow cooling treatment. The slow cooling temperature is 650-700℃, and the slow cooling rate is controlled at 0.5-5℃ / s. ③ Hot-dip aluminum-silicon plating: After slow cooling, the steel plate is put into an aluminum pot for hot-dip plating treatment. The temperature of the aluminum-silicon plating solution is 650-700℃, and the plating time is 3-10s. ④ After hot-dip aluminized silicon, cool to the quenching temperature of 320-360℃ at a cooling rate of more than 10℃ / s, then raise the temperature to the over-aging temperature of 380-460℃, the over-aging time is 30-650s, and then cool down to room temperature at a cooling rate of more than 2℃ / s. Finishing: The finishing elongation is controlled between 0.1% and 0.6%.

5. The method for preparing a hot-dip aluminized silicon 1470MPa grade Q&P cold-formed steel sheet according to claim 4, characterized in that: The composition of the plating solution in the aluminum pot, by weight percentage, is as follows: Al content 88%–93%, Si content 7%–12%.

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