980mpa grade hot-dip al-si high hole expansion multi-phase steel and preparation method thereof

Abstract

The application provides a 980MPa-grade hot-dip aluminized silicon high-hole-expanding multiphase steel plate and a manufacturing method thereof. The steel plate is composed of the following components in percentage by weight: C: 0.08% to 0.12%, Si: 0.1% to 0.6%, Mn: 1.9% to 2.7%, Cr: 0.1 to 0.6%, Mo: 0.05% to 0.3%, Ti: 0.01% to 0.04%, Nb: 0.01% to 0.04%, P: ≤0.02%, S: ≤0.005%, 2.4% ≤ Mn+Cr ≤ 3.2%, 1 / 3 ≤ Mo / Cr ≤ 2 / 3, and the balance of Fe and inevitable impurities. The aluminized silicon liquid of the steel plate is composed of the following components: Al content: 88% to 93%, and Si content: 7% to 12%. The plating liquid temperature is 650 to 700 DEG C. The steel material has a tensile strength of more than 980MPa, a yield strength of 780 to 950MPa, an elongation of ≥12%, and a hole-expanding rate of ≥60%. The steel plate is matched with a high-temperature slow cooling heat treatment process by using a novel hot-dip aluminized silicon process to achieve the corrosion resistance, breaks the limitation of high-temperature short-time overaging of the hot-dip galvanized steel plate, and obtains a novel multiphase steel with a tensile strength of more than 980MPa, a yield strength of 780 to 950MPa, an elongation of more than 12%, and a hole-expanding value of more than 60%, so that the good strength-ductility and formability of the steel plate are realized.

Description

Technical Field

[0001] This invention belongs to the field of automotive steel manufacturing and relates to a 980MPa grade hot-dip aluminized silicon high-expansion-hole multiphase steel and its preparation method. Background Technology

[0002] With increasingly stringent requirements for lightweighting and safety in the automotive industry, 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 third-generation high-strength steel. Among them, multiphase steel features high strength and good flanging performance, making it suitable for roll forming in automotive parts manufacturing processes. Multiphase 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, seat rails, and other parts, showing broad market prospects. Currently, the strength level of multiphase steel produced industrially internationally is mainly below 800MPa, with a few reaching 1000MPa, and only a small portion of these multiphase steel plates meeting high corrosion resistance requirements. Currently, high corrosion-resistant multiphase steel is mainly achieved through hot-dip galvanizing of steel plate surfaces. 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 multiphase steel with good corrosion resistance and mechanical properties.

[0003] Chinese invention patent application CN 113528978B discloses a 980MPa grade galvanized multiphase steel and its preparation method. The chemical composition of the multiphase steel is: C: 0.10%–0.12%, Si: 0.5%–1.0%, Mn: 2.0%–2.5%, Alt: 0.03%–0.1%, Ti: 0.03%–0.04%, P≤0.01%, S≤0.01%, N≤0.004%, with the balance being Fe and trace elements. The annealing temperature is 850–900℃, and the galvanizing temperature is 450–460℃, achieving corrosion resistance through galvanizing. The steel plate microstructure consists of bainite, martensite, ferrite, and retained austenite. The steel plate has a yield strength ≥780MPa, a tensile strength 980MPa, and an A50 ≥14%. The surface of the steel plate is not coated with aluminum-silicon.

[0004] Chinese invention patent application CN 109594020B discloses a cold-rolled multiphase steel with a tensile strength of 1000 MPa and its preparation method. The chemical composition of the multiphase steel, by mass percentage, is: C: 0.07-0.13%, Si: 0.2-0.4%, Mn: 1.8-2.5%, Cr: 0.2-0.6%, Mo: 0.1-0.4%, P: ≤0.02%, S: ≤0.015%, Nb: 0.02-0.06%, Ti: 0.02-0.06%, with the balance being iron and other unavoidable impurities. The annealing temperature is 800-850℃, followed by over-aging at 300-340℃ for 8-12 minutes. The heat-treated steel plate has a yield strength ≥780 MPa, a tensile strength ≥1000 MPa, and an elongation after fracture ≥8%. The aging temperature in this patent is relatively long, which places high demands on the steel mill's production line capacity, making it difficult for most steel mills to achieve. Furthermore, this steel plate does not involve any coating. Summary of the Invention

[0005] To address the shortcomings of existing technologies, this invention provides a 980MPa grade hot-dip aluminized silicon high-pore-expansion multiphase steel and its preparation method. Through the ingenious design of the continuous annealing aluminized silicon plating process and the rational control of the aluminum-silicon plating solution composition, the steel achieves a tensile strength ≥980MPa, a yield strength of 780~950MPa, an elongation ≥12%, and a pore-expansion rate ≥60%, thus realizing a high corrosion-resistant multiphase steel that simultaneously takes into account excellent mechanical properties and pore-expansion performance.

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

[0007] A 980MPa grade multiphase steel plate with an aluminum-silicon coating includes a substrate and a coating. The chemical composition of the substrate, by weight percentage, includes: C: 0.08%–0.12%, Si: 0.1%–0.6%, Mn: 1.9%–2.7%, Cr: 0.1%–0.6%, Mo: 0.05%–0.3%, Ti: 0.01%–0.04%, Nb: 0.01%–0.04%, P≤0.02%, S≤0.005%, with the balance being Fe and unavoidable impurities.

[0008] Furthermore, the chemical composition of the coating, by weight percentage, includes the following components: Al content 88-93%, Si content 7-12%.

[0009] Furthermore, the microstructure of the steel plate includes ferrite, bainite, tempered martensite, and martensite. Statistically, using the planar area method, the ferrite content is 10%–30%, the bainite content is 20%–80%, and the martensite content is 10%–50%.

[0010] Furthermore, the percentage is 2.4% ≤ Mn + Cr ≤ 3.2%.

[0011] Furthermore, the 1 / 3 ≤ Mo / Cr ≤ 2 / 3.

[0012] Furthermore, the steel plate has a tensile strength ≥980MPa, a yield strength of 780~950MPa, an elongation ≥12%, and a hole expansion rate ≥60%.

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

[0014] C: 0.08%~0.12%

[0015] Carbon (C) is a traditional and economical strengthening element for low-carbon steel. However, excessive C 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. Conversely, excessively low C content makes it difficult for the steel plate to reach 980 MPa. Therefore, this application controls the C content at around 0.1%, with the optimal range being 0.08–0.12%.

[0016] Si: 0.1%–0.6%

[0017] Si primarily strengthens ferrite, but excessive Si content can inhibit cementite precipitation during the over-aging stage, leading to the formation of retained austenite or high-hardness martensite, which is detrimental to the hole-expanding performance of the steel plate. Therefore, in this invention, the Si content is controlled at 0.1% to 0.6%.

[0018] Mn: 1.9%~2.7%, Cr: 0.1~0.6%

[0019] Mn and Cr are austenite stabilizing elements that significantly enhance the solid solution strengthening of high-strength steels. They substantially improve hardenability, refine ferrite grains, and significantly delay the transformation of pearlite and bainite, thereby increasing steel strength. They are the main strengthening elements besides C. Since the C content is fixed at around 0.1%, to achieve a steel plate strength of 980 MPa, the Mn+Cr content is controlled between 2.2% and 3.2%. Too low an Mn+Cr content may result in insufficient steel plate strength, while too high a content will reduce hole-expanding performance.

[0020] Mo: 0.05%–0.3%

[0021] Mo generally works better when added in combination with Cr, with the content being 1 / 3 to 2 / 3 of that of Cr.

[0022] Ti: 0.01%~0.04%

[0023] 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 the composition. Therefore, in this invention, the Ti content is controlled at 0.01%–0.04%.

[0024] Nb: 0.01%–0.04%

[0025] Nitrogen (Nb) plays a significant role in grain refinement, phase transformation behavior, carbon 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%.

[0026] P≤0.02%

[0027] Phosphorus (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 to be P ≤ 0.02%.

[0028] S≤0.005%

[0029] Sulfur (S) is a harmful element in steel, and its content should be as low as possible. Considering cost, the sulfur content in this invention is controlled to S ≤ 0.005%.

[0030] The second technical solution of this invention provides a method for manufacturing 980MPa grade hot-dip aluminized silicon high-hole-expansion multiphase steel, including a series of processes such as smelting, hot rolling, pickling, cold rolling, continuous annealing of aluminized silicon, and finishing. The specific steps of this preparation process are as follows:

[0031] Smelting: Smelting is carried out in a converter to obtain alloy compositions within the above range.

[0032] Hot rolling: ① Heating temperature between 1220 and 1260℃, holding time ≥ 2h, to ensure Ti atom precipitation, effectively consolidating N in the steel plate and ensuring Ti(C,N) precipitation, thus pinning the original austenite grain boundaries and refining the original austenite grains. ② Initial rolling temperature between 1100 and 1160℃, final rolling temperature 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 Coiling temperature between 560 and 700℃, to prevent excessively low coiling temperatures from increasing the difficulty of cold rolling. Hot-rolled coil thickness between 2.8 and 4.0 mm.

[0033] Pickling: Removes the iron oxide scale generated on the hot-rolled surface to ensure the surface quality of cold-rolled steel sheets.

[0034] 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.

[0035] Continuous annealing:

[0036] ① The heating isothermal temperature is 820-880℃, 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;

[0037] ② Aluminized silicon plating: After slow cooling, the steel plate is immersed in molten aluminum-silicon solution for 0-30 seconds at 650-700℃ to form a coating.

[0038] The composition of the aluminum-silicon plating solution on the steel plate is: Al content 88-93%, Si content 7-12%; the plating temperature is 650-700℃.

[0039] ③ After aluminizing with silicon, the steel plate is cooled to 300-420°C at a rate greater than 10°C / s, with an isothermal time of 200-650s. Then, it is cooled to room temperature at a rate greater than 2°C / s. The steel plate is then placed in a finishing machine for shape adjustment, and the finishing elongation is controlled at 0.1%-0.8%.

[0040] The mechanism is as follows: First, annealing is performed in the austenitic or two-phase region to obtain a suitable ratio of ferrite and original austenite; then, the mixture is slowly cooled to 650–700℃ for hot-dip aluminizing with silicon, followed by isothermal cooling to 300–420℃ to obtain a mixed microstructure of martensite / bainite / tempered martensite. 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 bainite transformation, which is beneficial to the pore-expanding performance and mechanical properties of the multiphase steel.

[0041] The beneficial effects of this invention are as follows:

[0042] (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 0.12%, which is beneficial to laser welding and resistance spot welding in the production and application process.

[0043] (2) The present invention adopts a novel hot-dip aluminum-silicon method to achieve high surface quality of multiphase steel, and cleverly performs hot-dip aluminum-silicon galvanization in the slow cooling stage, which perfectly matches the heat treatment regime, breaks the limitation that hot-dip galvanized steel sheets must be subjected to high temperature and short-time aging, and can make the bainite transformation sufficient, which is beneficial to the hole expansion performance and mechanical properties of multiphase steel.

[0044] (3) The final microstructure of the steel plate of the present invention is 10% to 30% ferrite, 20% to 80% bainite, 10% to 50% martensite, and the surface of the steel plate is an aluminum-silicon coating.

[0045] (4) This invention achieves excellent strength, formability, and high surface quality of steel plates through low-cost alloy design and ingenious process design. The invention uses a novel hot-dip galvanizing process matched with a heat treatment process to obtain a new type of multiphase steel with a tensile strength of over 980 MPa, a yield strength of 780-950 MPa, an elongation of over 12%, and a hole expansion value of over 60%, thus achieving excellent strength, formability, and high surface quality of steel plates. Attached Figure Description

[0046] Figure 1 This is a typical SEM metallographic image of Embodiment 1 of the present invention. Detailed Implementation

[0047] The present invention will be further illustrated below through examples.

[0048] 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 and finishing.

[0049] Hot rolling

[0050] The heating temperature is between 1220 and 1260℃, and the holding time is ≥2h; the initial rolling temperature is between 1100 and 1160℃, and the final rolling temperature is above 900℃; the coiling temperature is between 560 and 700℃, and the thickness of the hot-rolled coil is between 2.8 and 4.0mm.

[0051] Cold rolling

[0052] The cold rolling reduction rate is 50%–58%;

[0053] Continuous annealing:

[0054] ① The heating isothermal temperature is 820-880℃, 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;

[0055] ② Aluminized silicon plating: After slow cooling, the steel plate is immersed in molten aluminum-silicon solution for 0-30 seconds at 650-700℃ to form a coating.

[0056] ③ After aluminum-silicon plating, cool to 300-420℃ at a cooling rate of more than 10℃ / s, with an isothermal time of 200-650s, and then cool to room temperature at a cooling rate of more than 2℃ / s.

[0057] ④ Finishing: The finishing elongation rate is controlled between 0.1% and 0.8%.

[0058] Furthermore, the composition of the aluminum-silicon plating solution on the steel plate is: Al content 88-93%, Si content 7-12%; the plating temperature is 650-700℃.

[0059] 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.

[0060] Table 1 lists the chemical composition of the steels in the examples, Table 2 lists the continuous casting and hot rolling process parameters of the steels in the examples, Table 3 lists the cold rolling and continuous annealing process parameters of the steels in the examples, and Table 4 gives the mechanical properties of the steels in the examples.

[0061] Table 1 Chemical composition of the steel in the examples, wt%.

[0062]

[0063] Table 2. Continuous casting and hot rolling processes for steel in the examples.

[0064]

[0065] Table 3. Steel Annealing Process in Examples

[0066]

[0067] Table 4 Mechanical properties and microstructure of the steels in the examples

[0068]

[0069] As can be seen from the above, the chemical composition of the steel plate of this invention is mainly composed of C and Mn, with no obvious precious alloys. Simultaneously, the C content is less than 0.12%, which is beneficial for laser welding and resistance spot welding during production and application. This invention employs a novel hot-dip aluminized silicon method to achieve good corrosion resistance in multiphase steel, and cleverly performs hot-dip aluminized silicon during the slow cooling stage, perfectly matching the heat treatment regime. This breaks the limitation that hot-dip galvanized steel plates must undergo high-temperature short-time over-aging, allowing for sufficient bainite transformation, which is beneficial for the hole-expanding performance and mechanical properties of multiphase steel. This invention achieves good strength, plasticity, and formability of the steel plate through a combination of low-cost alloy design and ingenious process design, meeting the forming requirements of complex parts for automotive steel. The prepared hot-dip aluminized silicon steel plate has a tensile strength of over 980 MPa, a yield strength of 780–950 MPa, an elongation greater than 12%, and a hole-expanding value of over 60%, achieving good strength, plasticity, formability, and corrosion resistance.

[0070] 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 980 MPa grade hot dip aluminum-silicon high hole expansion multiphase steel sheet comprising a base sheet and a plated layer, characterized in that, The chemical composition of the substrate, by weight percentage, includes the following components: C: 0.08%–0.095%, Si: 0.1%–0.6%, Mn: 2.3%–2.7%, Cr: 0.1%–0.6%, Mo: 0.05%–0.3%, Ti: 0.01%–0.04%, Nb: 0.01%–0.0025%, P≤0.02%, S≤0.005%, with the balance being Fe and unavoidable impurities; the microstructure of the steel plate includes ferrite, bainite, and martensite, with a ferrite content of 10%–17%, a bainite content of 45%–80%, and a martensite content of 10%–35% as calculated by the planar area method. The manufacturing method of a 980MPa grade hot-dip aluminized silicon high-perforation multiphase steel plate includes smelting, hot rolling, pickling, cold rolling, continuous annealing with aluminized silicon, and finishing. Hot rolling Heating temperature is between 1220~1260℃, holding time ≥2h; initial rolling temperature is between 1100~1160℃, final rolling temperature is above 900℃; coiling temperature is between 560~700℃, and hot-rolled coil thickness is between 2.8~4.0mm. cold rolling The cold rolling reduction rate is 50%~58%; Continuous annealing of aluminum-silicon plating: ① The heating isothermal temperature is 820~880℃, the isothermal time is 80~150s, the slow cooling temperature is 650~700℃, and the slow cooling rate is controlled at 0.9~4.2℃ / s; ② Aluminized silicon plating: After slow cooling, the steel plate is immersed in molten aluminum-silicon solution for 0-30 seconds at 650-700℃ to form a coating. ③ After aluminum-silicon plating, cool to 370~420℃ at a cooling rate greater than 27℃ / s, with an isothermal time of 200~650s, and then cool to room temperature at a cooling rate greater than 2℃ / s. ④ Finishing: The elongation rate of the finishing process is controlled between 0.1% and 0.8%; The chemical composition of the coating, by weight percentage, includes the following components: Al content 88~93%, Si content 7~12%.

2. The 980 MPa grade hot dip aluminized silicon high hole expansion multiphase steel plate according to claim 1, characterized in that, 2.4%≤Mn+Cr≤3.2%.

3. The 980 MPa grade hot dip aluminized silicon high hole expansion multiphase steel plate according to claim 1, characterized in that, 1 / 3 ≤ Mo / Cr ≤ 2 / 3.

4. The 980 MPa grade hot dip aluminized silicon high hole expansion multiphase steel plate according to claim 1, characterized in that, The steel plate has a tensile strength ≥980MPa, a yield strength of 780~950MPa, an elongation ≥12%, and a hole expansion rate ≥60%.

5. The 980 MPa grade hot dip aluminized silicon high hole expansion multiphase steel plate according to claim 1, characterized in that, The composition of the aluminum-silicon plating solution on the steel plate is: Al content 88~93%, Si content 7~12%; the plating temperature is 650~700℃.

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