800mpa grade hot-dip al-si high hole expansion multi-phase steel and preparation method thereof

Abstract

The application provides 800MPa-grade hot-dip aluminized silicon high-hole-expanding multiphase steel and a preparation method thereof. The composition of the steel plate is as follows in terms of percentage by weight: C: 0.06% to 0.09%, Si: 0.1% to 0.4%, Mn: 1.8% to 2.5%, Cr: 0.1% to 0.4%, Mo: 0.05% to 0.3%, Ti: 0.01% to 0.03%, Nb: 0.01% to 0.03%, P: less than or equal to 0.02%, S: less than or equal to 0.005%, 1 / 3: less than or equal to Mo / Cr: less than or equal to 2 / 3, 2.2%: less than or equal to Mn+Cr: less than or equal to 2.8%, and the balance of Fe and inevitable impurities. The chemical composition of the coating includes the following components in terms of percentage by weight: Al content: 88% to 93%, and Si content: 7% to 12%. The production method of the steel plate comprises a series of processes such as smelting, hot rolling, pickling, cold rolling, continuous annealing and aluminized silicon plating, and the structure of the steel plate comprises ferrite, bainite and martensite. In terms of the planar area method, the ferrite content is 18% to 35%, the bainite content is 35% to 62%, and the martensite content is 10% to 40%. The mechanical properties of the steel plate of the application are as follows: tensile strength is more than 800MPa, yield strength is 620MPa to 750MPa, elongation is greater than 14%, and the hole-expanding value is more than 50%, so that the steel plate has good strength and plasticity and formability, and meanwhile, the hole-expanding performance is excellent.

Description

Technical Field

[0001] This invention belongs to the field of automotive steel manufacturing and relates to an 800MPa grade hot-dip aluminized silicon high-expansion-hole multiphase steel and its preparation method. Background Technology

[0002] Currently, automobile fuel consumption is closely related to vehicle weight. Studies show that for every 10% reduction in vehicle weight, fuel consumption decreases by 6% to 10%, and emissions decrease by 4%. According to the fifth edition of the Advanced High-Strength Steel Application Guide published by the International Iron and Steel Institute, advanced high-strength steel generally refers to high-strength steel with a yield strength exceeding 550 MPa, while steel with a tensile strength exceeding 780 MPa is sometimes referred to as ultra-high-strength steel. Multiphase steel, as a typical representative of advanced high-strength steel, has a microstructure consisting of ferrite and bainite, and may contain martensite and austenite. Due to its excellent stamping and flanging properties, as well as good strength and plasticity, it is currently widely used in the automotive industry. Through the combined effects of martensite, bainite, and precipitation strengthening, multiphase steel possesses high strength and good flanging performance, making it suitable for roll forming in automotive parts forming processes and possessing broad market prospects. In addition to good mechanical and application properties, many car body components require good surface quality. Currently, the surface coating of multiphase steel is mainly hot-dip galvanized. 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 surface quality and mechanical properties.

[0003] Chinese patent application CN111926247 A discloses a hot-dip galvanized multiphase steel with a tensile strength of 800 MPa and its manufacturing method. This patent uses hot-dip galvanizing to obtain the surface coating, employing an annealing temperature of 760–840°C, an over-aging temperature of 450–460°C, and an over-aging time of 10–20 seconds during the continuous annealing process. After heat treatment, the resulting steel plate has a yield strength ≥660 MPa, a tensile strength ≥800 MPa, and a hole expansion rate between 20% and 55%. This patent does not involve an aluminum-silicon coating and exhibits poor hole expansion performance.

[0004] Chinese patent application CN113481436 A discloses an 800MPa grade hot-rolled multiphase steel and its production method. This patent uses a C, Mn, Si+Al composition design, and adds Cr to improve the hardenability of the steel plate and promote bainite production. The hot-rolled steel plate is obtained through smelting and hot rolling. The final steel plate has a tensile strength ≥800MPa, a yield strength ≥550MPa, an elongation ≥20%, and a thickness of 2.0–5.0mm. This steel plate is hot-rolled and does not involve cold rolling or surface coating treatment. Summary of the Invention

[0005] To address the shortcomings of existing technologies, this invention provides an 800MPa grade hot-dip aluminized silicon high-hole-expansion multiphase steel and its preparation method. This invention employs a novel hot-dip coating process matched with a heat treatment process, resulting in a novel multiphase steel with a tensile strength of over 800MPa, a yield strength of 620-750MPa, an elongation of over 14%, and a hole-expansion value of over 50%, thus achieving a multiphase steel that simultaneously possesses excellent mechanical properties and hole-expansion performance.

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

[0007] An 800MPa 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: 0.06%–0.09%, Si: 0.1%–0.4%, Mn: 1.8%–2.5%, Cr: 0.1%–0.4%, Mo: 0.05%–0.3%, Ti: 0.01%–0.03%, Nb: 0.01%–0.03%, 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 steel plate has a ratio of 1 / 3 ≤ Mo / Cr ≤ 2 / 3.

[0010] Furthermore, the steel plate contains 2.2% ≤ Mn + Cr ≤ 2.8%.

[0011] Furthermore, the microstructure of the steel plate includes ferrite, bainite and martensite, and according to the planar area method, the ferrite content is 18% to 35%, the bainite content is 35% to 62%, and the martensite content is 10% to 40%.

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

[0013] C: 0.06%~0.09%

[0014] 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 800 MPa. Therefore, this application controls the C content at around 0.08%, with the optimal range being 0.06% to 0.09%.

[0015] Si: 0.1%–0.4%

[0016] 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 0.4%.

[0017] Mn: 1.8%~2.5%, Cr: 0.1%~0.4%

[0018] 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 0.08%, the Mn+Cr content is controlled between 2.0% and 2.8% to achieve a steel plate strength of 800 MPa. Too low an Mn+Cr content may result in insufficient steel plate strength, while too high a content will reduce hole-expanding performance.

[0019] Mo: Mo: 0.05%–0.3%

[0020] Mo mainly plays a role in improving the hardenability of steel plates, and it generally works better when added in combination with Cr. Therefore, this invention requires the content of Mo to be 0.05% to 0.3%, and its content is preferably 1 / 3 to 2 / 3 of that of Cr.

[0021] Ti: 0.01%~0.03%

[0022] 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%–0.03%.

[0023] Nb: 0.01%–0.03%

[0024] 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 Mn content to 0.01–0.03%.

[0025] P≤0.02%

[0026] Phosphorus (P) is a harmful element in steel, and its content should be as low as possible. In this invention, the P content is controlled to be P ≤ 0.02%.

[0027] S≤0.005%

[0028] Sulfur (S) is a harmful element in steel, and its content should be as low as possible. In this invention, the S content is controlled to be ≤0.005%.

[0029] The second technical solution of this invention provides an 800MPa grade hot-dip aluminized silicon high-permeability multiphase steel and its preparation method, characterized by comprising the following steps: smelting, hot rolling, pickling, cold rolling, continuous annealing, and finishing. The specific steps of this preparation process are as follows:

[0030] Smelting: Smelting is carried out in a converter to obtain alloy compositions within the above range.

[0031] Hot rolling: ① Heating temperature is between 1200 and 1250℃, and holding time is ≥100 min. This limited heating temperature and holding time ensure the precipitation of Ti atoms, effectively consolidating N in the steel plate and guaranteeing the precipitation of Ti(C,N), thus pinning the original austenite grain boundaries and refining the original austenite grains. ② Initial rolling temperature is between 1100 and 1150℃, and 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 Coiling temperature is between 660 and 700℃ to prevent excessively low coiling temperatures from increasing the difficulty of cold rolling. The thickness of the hot-rolled coil is between 2.8 and 4.0 mm.

[0032] Pickling: Removes the iron oxide scale generated on the hot-rolled surface to ensure the surface quality of cold-rolled steel sheets.

[0033] 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.

[0034] Continuous annealing: ① Heat to annealing temperature of 820-880℃, annealing time of 80-150s, slow cooling temperature of 650-700℃, and slow cooling rate controlled at 0.5-5℃ / s; Heating the strip to this annealing temperature and annealing for 80-150s can effectively cause the cold-rolled structure to recrystallize and place the strip in the ferrite and austenite two-phase region or the austenite region, which can make the elements in the steel diffuse evenly; The slow cooling temperature and slow cooling rate can make some of the austenite in the steel plate form ferrite, and adjust the final microstructure of the strip.

[0035] ② Aluminized silicon plating: After slow cooling, the strip steel enters the aluminum-silicon plating bath to obtain a steel plate coating. The temperature of the aluminum-silicon plating bath is 650-700℃; the aluminum-silicon plating time is 3-10s; the composition of the aluminum-silicon plating bath on the steel plate is: Al content 88-93%, Si content 7-12%.

[0036] ③ After aluminized silicon coating, the steel plate is cooled to an over-aging temperature of 300-420℃ at a rapid cooling rate of more than 10℃ / s, and the over-aging isothermal time is 200-650s. Then, it is cooled to room temperature at an over-aging cooling rate of more than 2℃ / s. Finally, the steel plate enters the finishing machine for plate shape adjustment, and the finishing elongation is controlled at 0.1%-0.8%.

[0037] 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°C for hot-dip aluminizing with silicon, followed by isothermal cooling to 300–420°C to obtain a martensite + bainite microstructure. Because the hot-dip aluminizing with silicon occurs during the slow cooling stage, the aging 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 aging. This allows for sufficient bainite transformation, which is beneficial to the hole-expanding performance and mechanical properties of the multiphase steel.

[0038] The steel plate contains 18%–35% ferrite, 35%–62% bainite, and 10%–40% martensite, with an aluminum-silicon coating on its surface. It is evident that this invention employs a novel hot-dip galvanizing process combined with a heat treatment process, resulting in a novel multiphase steel with a tensile strength exceeding 800 MPa, a yield strength of 620–750 MPa, an elongation greater than 14%, and a hole expansion value exceeding 50%, achieving excellent strength, plasticity, and formability of the steel plate.

[0039] The beneficial effects of this invention are as follows:

[0040] (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 0.09%, which is beneficial to laser welding and resistance spot welding in the production and application process.

[0041] (2) The present invention uses a novel hot-dip aluminum-silicon method to obtain the coating of multiphase steel, and cleverly performs hot-dip aluminum-silicon 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 hole expansion performance and mechanical properties of multiphase steel.

[0042] (3) The microstructure of the steel plate of the present invention includes ferrite, bainite and martensite. The ferrite content is 18% to 35%, the bainite content is 35% to 62% and the martensite content is 10% to 40% according to the planar area method.

[0043] (4) This invention achieves excellent strength, plasticity, and formability of the steel plate through low-cost alloy design and ingenious process design. The tensile strength of the steel is above 800MPa, the yield strength is 620-750MPa, the elongation is ≥14%, and the hole expansion rate is ≥50%. Attached Figure Description

[0044] Figure 1This is a typical SEM metallographic image of the tissue in Example 1 of the present invention. Detailed Implementation

[0045] The present invention will be further illustrated below through examples.

[0046] According to the component ratio of the technical solution, the embodiments of the present invention carry out alloying smelting, hot rolling, pickling, cold rolling, continuous annealing aluminum-silicon plating, and finishing.

[0047] Hot rolling: heating temperature between 1200 and 1250℃, holding time ≥ 100 min; initial rolling temperature between 1100 and 1150℃, final rolling temperature above 900℃, coiling temperature between 660 and 700℃, hot rolled coil thickness 2.8 to 4.0 mm.

[0048] Cold rolling: The cold rolling reduction rate is 50% to 58%;

[0049] Continuous annealing of aluminum-silicon plating:

[0050] ① Heat to annealing temperature of 820-880℃, annealing time of 80-150s, slow cooling temperature of 650-700℃, and slow cooling rate controlled at 0.5-5℃ / s;

[0051] ② Aluminized silicon plating: After slow cooling, the steel plate is immersed in an aluminum-silicon plating solution to obtain a coating on the surface. The temperature of the aluminum-silicon plating solution is 650-700℃, and the plating time is 3-10s.

[0052] ③ After aluminized silicon coating, the steel plate is cooled to the over-aging temperature of 300-420℃ at a rapid cooling rate of more than 10℃ / s, and the over-aging isothermal time is 200-650s. Then, it is cooled to room temperature at a final cooling rate of more than 2℃ / s.

[0053] Furthermore, after continuous annealing, the steel sheet enters a finishing machine for shape adjustment, with a finishing elongation of 0.1% to 0.8%.

[0054] Furthermore, the composition of the aluminum-silicon plating solution is: Al content 88-93%, Si content 7-12%.

[0055] 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 annealing and aluminizing / silicon coating of the steel in this embodiment of the invention are shown in Table 3. The properties and microstructure of the steel in this embodiment of the invention are shown in Table 4.

[0056] Table 1 Chemical composition of the steel in the examples, wt%.

[0057] Example C Mn Cr Mo Si Ti Nb P S Mn+Cr Mo / Cr 1 0.09 1.9 0.4 0.2 0.3 0.02 0.015 0.010 0.005 2.3 1 / 2 2 0.08 2.3 0.2 0.1 0.2 0.02 - 0.009 0.005 2.5 1 / 2 3 0.065 2.4 0.4 0.2 0.4 0.015 0.015 0.010 0.003 2.8 1 / 2 4 0.075 2.3 0.2 0.1 0.4 0.02 0.02 0.005 0.005 2.5 1 / 2 5 0.08 2.1 0.3 0.2 0.4 0.02 - 0.009 0.003 2.4 2 / 3 6 0.085 2.2 0.3 0.1 0.2 0.015 0.015 0.008 0.005 2.5 1 / 3 7 0.09 2.4 0.3 0.2 0.4 0.02 - 0.01 0.005 2.7 2 / 3 8 0.080 2.3 0.4 0.2 0.2 0.015 0.025 0.02 0.004 2.7 1 / 2

[0058] Table 2. Continuous casting and hot rolling processes for steel in the examples.

[0059]

[0060] Table 3. Steel Annealing Process in Examples

[0061]

[0062] Table 4 Mechanical properties and microstructure of the steels in the examples

[0063]

[0064] As can be seen from the above embodiments, through low-cost alloy design and ingenious process design, the microstructure of the hot-dip aluminized silicon steel sheet includes ferrite, bainite, and martensite. Statistically, using the planar area method, the ferrite content is 18%–35%, bainite 35%–62%, and martensite 10%–40%. The mechanical properties of the steel sheet of this invention are tensile strength above 800 MPa, yield strength 620–750 MPa, elongation greater than 14%, and hole expansion value above 50%, achieving good strength, plasticity, and formability of the steel sheet while also taking into account excellent hole expansion performance.

[0065] 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 method for manufacturing 800MPa grade hot-dip aluminized silicon high-expansion multiphase steel, comprising the following steps: Smelting, hot rolling, pickling, cold rolling, continuous annealing, aluminizing and silicon plating, and finishing, characterized in that, Hot-rolled: Heating temperature is between 1200~1250℃, holding time ≥100 min; initial rolling temperature is between 1100~1150℃, final rolling temperature is above 900℃; coiling temperature is between 660~700℃, and hot-rolled coil thickness is between 2.8~4.0mm. Cold rolling: The cold rolling reduction rate is 50%~58%; Continuous annealing aluminum-silicon plating: ① The heating annealing temperature is 820~880℃, the annealing time is 80~150s, the slow cooling temperature is 650~700℃, and the slow cooling rate is controlled at 0.5~5℃ / s; ② Aluminized silicon plating: After slow cooling, the steel plate is immersed in an aluminum-silicon plating solution to obtain a coating on the surface. 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 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 final cooling rate of more than 2℃ / s. The aforementioned 800MPa grade hot-dip aluminized silicon high-expansion multiphase steel comprises a substrate and a coating. The chemical composition of the substrate, by weight percentage, includes the following components: C: 0.06%–0.09%, Si: 0.1%–0.4%, Mn: 1.8%–2.5%, Cr: 0.1%–0.4%, Mo: 0.05%–0.3%, Ti: 0.01%–0.03%, Nb: 0.01%–0.03%, P≤0.02%, S≤0.005%, with the balance being Fe and unavoidable impurities.

2. The method for manufacturing an 800MPa grade hot-dip aluminized silicon high-expansion multiphase steel according to claim 1, characterized in that, The chemical composition of the coating, by weight percentage, includes the following components: Al content 88%~93%, Si content 7%~12%.

3. The method for manufacturing an 800MPa grade hot-dip aluminized silicon high-expansion multiphase steel according to claim 1, characterized in that, 1 / 3 ≤ Mo / Cr ≤ 2 / 3.

4. The manufacturing method of an 800MPa grade hot-dip aluminized silicon high-expansion multiphase steel according to claim 1, characterized in that, 2.2%≤Mn+Cr≤2.8%.

5. The method for manufacturing an 800MPa grade hot-dip aluminized silicon high-expansion multiphase steel according to claim 1, characterized in that, The steel has a tensile strength ≥800MPa, a yield strength of 620~750MPa, an elongation ≥14%, and a hole expansion rate ≥50%.

6. The method for manufacturing an 800MPa grade hot-dip aluminized silicon high-expansion multiphase steel according to claim 1, characterized in that, The microstructure of the steel plate includes ferrite, bainite and martensite. According to the planar area method, the ferrite content is 18%~35%, the bainite content is 35%~62%, and the martensite content is 10%~40%.

7. The method for manufacturing an 800MPa grade hot-dip aluminized silicon high-expansion multiphase steel according to claim 1, characterized in that, After continuous annealing, the steel sheet enters the finishing machine for shape adjustment, with a finishing elongation of 0.1%~0.8%.

8. The method for manufacturing an 800MPa grade hot-dip aluminized silicon high-expansion multiphase steel according to claim 1, characterized in that, The composition of the aluminum-silicon plating solution is: Al content 88~93%, Si content 7~12%.

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