600mpa grade hot-dip al-si high hole expansion multi-phase steel and preparation method thereof

Abstract

The application provides a 600MPa-grade hot-dip aluminized silicon high-hole-expanding multiphase steel and a preparation method thereof. The steel plate is composed of the following components in percentage by weight: C: 0.08%-0.12%, Si: 1.0%-2.0%, Mn: 0.2%-0.5%, Cr: 0.1-0.4%, Mo: 0.05-0.2%, P: less than or equal to 0.02%, S: less than or equal to 0.005%, Ti: 0.01-0.04%, Nb: 0.01-0.03%, Mn+Cr: 0.5%-0.8%, Mo / Cr: 1 / 3-2 / 3, and the balance of Fe and inevitable impurities. The aluminized silicon liquid for the steel plate is composed of the following components: Al: 88-93%, and Si: 7-12%. The production method of the steel plate comprises the following steps: smelting, hot rolling, pickling, cold rolling, continuous annealing, aluminizing, and finishing. The final performance of the 600MPa-grade hot-dip aluminized silicon high-hole-expanding multiphase steel plate is as follows: tensile strength: more than 600MPa, yield strength: 400-550MPa, elongation: greater than 20%, and hole-expanding value: more than 90%. The microstructure of the steel plate is composed of ferrite, bainite, martensite and pearlite, and the content of each component is as follows in percentage by area: ferrite: 35%-61%, bainite: 20%-35%, pearlite: 6%-12%, and martensite: 12%-25%. The steel plate realizes good strength-plasticity and formability, and has excellent hole-expanding performance.

Description

Technical Field

[0001] This invention belongs to the field of automotive steel manufacturing, and specifically relates to a 600MPa grade hot-dip aluminized silicon high-expansion-hole multiphase steel and its preparation method. Background Technology

[0002] 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 typically refers to high-strength steel with a strength exceeding 550 MPa. Multiphase steel, a typical representative of advanced high-strength steel, has a microstructure consisting of ferrite and bainite, possibly containing 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. Besides good mechanical and application properties, many vehicle body components require good surface quality. Currently, the surface coating of multiphase steel is mainly hot-dip galvanizing. 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 No. CN111945075B discloses a 590MPa grade high-hole-expansion performance alloyed hot-dip galvanized DH steel and its preparation method. It employs a C, Mn, Si+Al composition system, adding 0.02–0.6% Al to suppress the decomposition of residual austenite and the precipitation of carbides. However, the addition of Al can hinder continuous casting during the steelmaking process. Furthermore, it uses alloyed zinc plating to obtain the coating, with an alloying temperature between 470 and 530°C and an alloying holding time of 5–60 seconds. This high-temperature, short-duration alloying process is not conducive to the full bainitic transformation, potentially affecting the hole-expansion performance of the steel plate.

[0004] Chinese patent CN103667948B discloses a multiphase steel and its preparation method. This invention uses a composition design of C, Mn, Si+Al, and adds Cr to increase the hardenability and refine the microstructure. The steel plate is produced through smelting and hot rolling, achieving a yield strength of 430–590 MPa, a tensile strength of 600–700 MPa, an elongation of 20–35%, and a hole expansion rate of over 80%. However, this invention does not involve cold rolling or coating. Summary of the Invention

[0005] To address the shortcomings of existing technologies, the present invention aims to provide a method for preparing 600MPa grade multiphase steel plates with aluminum-silicon coatings. By ingeniously designing the composition and process, and rationally controlling the aluminum-silicon liquid, and by employing a novel hot-coating process matched with a heat treatment process, a novel multiphase steel is obtained with a tensile strength of over 600MPa, a yield strength of 400-550MPa, an elongation of over 20%, and a hole expansion value of over 90%. This achieves good strength, plasticity, and formability of the steel plate, while also taking into account excellent hole expansion performance, resulting in aluminum-silicon coated multiphase steel.

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

[0007] This invention provides a 600MPa grade multiphase steel plate with an aluminum-silicon coating, comprising a substrate and a coating. The chemical composition of the substrate, by weight percentage, includes: C: 0.08%–0.12%, Si: 1.0%–2.0%, Mn: 0.2%–0.5%, Cr: 0.1%–0.4%, Mo: 0.05%–0.2%, P≤0.02%, S≤0.005%, Ti: 0.01%–0.03%, Nb: 0.01%–0.03%, 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 substrate contains Mn+Cr = 0.5% to 0.8%.

[0010] Furthermore, the Mo / Cr ratio in the substrate is 1 / 3 to 2 / 3.

[0011] Furthermore, the steel plate has a tensile strength of ≥600MPa, a yield strength of 400-550MPa, an elongation of ≥20%, and a hole expansion rate of ≥90%.

[0012] Furthermore, the microstructure of the steel plate includes ferrite, bainite, pearlite and martensite. Statistically, using the planar area method, the ferrite content is 35%–61%, the bainite content is 20%–35%, the pearlite content is 6%–12%, and the martensite content is between 12% and 25%.

[0013] The rationale for the alloy design of 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 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 600 MPa. Therefore, this application controls the carbon content at about 0.1%, with the optimal range being 0.08 to 0.12%.

[0015] Si: Si mainly plays a role in strengthening ferrite, which helps to reduce the hardness difference between phases and thus improves the hole expansion performance of the steel plate. Therefore, in this invention, the content of Si is controlled at 1.0% to 2.0%.

[0016] Mn: Mn is a strengthening element for steel plates, increasing their strength. It is also an austenite stabilizing element, improving the hardenability of the steel plate. Too low a content of Mn affects the strength of the steel plate, while too high a content can lead to segregation and deteriorate its performance. Therefore, this invention requires the content of Mn to be controlled between 0.2% and 0.5%.

[0017] Cr: Similar to Mn, Cr can strengthen steel plates through solid solution treatment. It is also a stabilizing element for austenite, significantly delaying the transformation of pearlite and bainite, and thus significantly improving the hardenability of steel. Furthermore, since the C content is fixed at approximately 0.1% in this invention, to achieve a steel plate strength of 600 MPa, the Cr content must be controlled between 0.1% and 0.4%. At the same time, too low a Mn+Cr content may result in insufficient steel plate strength, while too high a content will reduce the hole-expanding performance; therefore, the Mn+Cr content is also required to be between 0.5% and 0.8%.

[0018] Mo: Mo mainly plays a role in improving the hardenability of steel plates and inhibiting the occurrence of ferrite and bainite phase transformations. It generally works well when added in combination with Cr. Too low a content of this element will result in insufficient hardenability of the steel plate, while too high a content will significantly increase the cost of the steel plate. Therefore, this invention requires that the content of Mo be controlled at 0.05% to 0.2%, and that its content be 1 / 3 to 2 / 3 of that of Cr.

[0019] 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 the composition. Therefore, in this invention, the Ti content is controlled at 0.01%–0.03%.

[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 Mn content to 0.01–0.03%.

[0021] P: P is a harmful element in steel. The lower its content, the better. However, excessively low content will increase the difficulty of steelmaking and increase smelting costs. Therefore, in this invention, the content of P is controlled at P≤0.02%.

[0022] S: S is a harmful element in steel, and its content should be as low as possible. However, excessively low S content will significantly increase the difficulty of steelmaking and thus increase costs. Therefore, in this invention, the S content is controlled at S≤0.005%.

[0023] The second technical solution of this invention provides a method for preparing 600MPa grade hot-dip aluminized silicon high-hole-expansion multiphase steel, comprising the following steps: smelting, hot rolling, pickling, cold rolling, continuous annealing with aluminized silicon, and finishing, as detailed below:

[0024] Smelting: Smelting is carried out in a converter to obtain alloy compositions within the above range.

[0025] Hot rolling: ① Heating temperature is between 1200 and 1250℃, and holding time is ≥100 min. This limited temperature and time ensures that Ti atoms precipitate with N and C elements in the steel to form Ti(C,N), thus effectively fixing N elements in the steel. Simultaneously, the precipitation of Ti(C,N) helps to pin the original austenite grain boundaries and refine 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 560 and 700℃ to prevent excessively low coiling temperatures from increasing the difficulty of cold rolling. The thickness of the hot-rolled coil is between 2.5 and 4.0 mm.

[0026] Pickling: Removes the iron oxide scale generated on the hot-rolled surface to ensure the surface quality of cold-rolled steel sheets.

[0027] Cold rolling: The cold rolling reduction rate is 40% to 60%, ensuring a rolling reduction of more than 40% to promote the fibrosis 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.

[0028] Continuous annealing aluminum-silicon plating:

[0029] ① The heating isothermal temperature is 760-840℃, the isothermal time is 80-150s, the slow cooling temperature is 650-700℃, and the slow cooling rate is controlled at 0.5-5℃ / s. This heating temperature and isothermal time ensure that the cold-rolled structure can recover and recrystallize, forming a two-phase structure of ferrite and austenite, and ensuring the homogenization of austenite composition. Ferrite and pearlite will be generated during the slow cooling process. This process promotes the enrichment of C atoms into the supercooled austenite.

[0030] ② Aluminized silicon plating: After slow cooling, the strip steel enters the aluminum-silicon plating bath to obtain a surface coating. The temperature of the aluminum-silicon plating bath is 650-700℃, and the plating time is 3-10s. The composition of the aluminum-silicon plating bath is: Al content 88-93%, Si content 7-12%.

[0031] ③ After aluminizing with silicon, the steel plate is cooled to the over-aging temperature of 320-420℃ at a 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 cooling rate of more than 2℃ / s. Then, the steel plate enters the finishing machine for plate shape adjustment, which is controlled at 0.1%-0.8%.

[0032] 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 320–420°C to obtain martensite and bainite structures. 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.

[0033] The final microstructure of the steel plate of this invention consists of ferrite, bainite, martensite, and pearlite, with an aluminum-silicon coating on the 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 600 MPa, a yield strength of 400–550 MPa, an elongation greater than 20%, and a hole expansion value exceeding 90%, achieving excellent strength, plasticity, and formability of the steel plate.

[0034] The beneficial effects of this invention are as follows:

[0035] 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, and the C content is less than 0.12%, which is beneficial for laser welding and resistance spot welding in the production and application process;

[0036] 2. This invention employs a novel hot-dip aluminizing-silicon method to achieve a coating on the surface of multiphase steel, and cleverly performs hot-dip aluminizing-siliconization during the slow cooling stage, perfectly matching the heat treatment regime. This breaks the limitation that hot-dip galvanized steel sheets must undergo high-temperature short-time aging, allowing for sufficient bainite transformation, which is beneficial to the hole expansion performance and mechanical properties of multiphase steel.

[0037] 3. This invention achieves excellent strength, plasticity, and formability of the steel plate through low-cost alloy design and ingenious process design. The final microstructure of the steel plate of this invention is ferrite + bainite + martensite + pearlite, with ferrite content of 35%–61%, bainite content of 20%–35%, pearlite content of 6%–12%, and martensite content between 12% and 25%. The surface of the steel plate is coated with an aluminum-silicon coating.

[0038] 4. The present invention employs a novel hot-dip galvanizing process and a heat treatment process to obtain a new type of multiphase steel with a tensile strength ≥600MPa, a yield strength of 400~550MPa, an elongation ≥20%, and a hole expansion value ≥90%, thus achieving good strength, plasticity and formability of the steel plate. Attached Figure Description

[0039] Figure 1 This is a typical SEM metallographic image of the tissue in Example 1 of the present invention. Detailed Implementation

[0040] The present invention will be further illustrated below through examples.

[0041] 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.

[0042] 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 560 and 700℃, hot rolled coil thickness 2.5 to 4.0 mm.

[0043] Cold rolling: The cold rolling reduction rate is 40% to 60%;

[0044] Continuous annealing of aluminum-silicon plating:

[0045] ① The heating isothermal temperature is 760-840℃, 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;

[0046] 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.

[0047] ③ After aluminum-silicon plating, cool to the over-aging temperature of 320-420℃ at a cooling rate of greater than 10℃ / s, and the over-aging isothermal time is 200-650s, then cool to room temperature at a cooling rate of greater than 2℃ / s.

[0048] Furthermore, after continuous annealing, the steel sheet enters the finishing machine for shape adjustment, with a finishing elongation of 0.1% to 0.8%.

[0049] Furthermore, the composition of the aluminum-silicon plating solution on the steel plate is: Al content 88-93%, Si content 7-12%.

[0050] 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.

[0051] Table 1 Chemical composition of the steel in the examples, wt%.

[0052] No. C Mn Cr Mo Si Ti Nb P S Mn+Cr Mo / Cr 1 0.09 0.5 0.2 0.1 1.6 0.02 0.015 0.010 0.005 0.7 0.5 2 0.08 0.3 0.3 0.1 1.2 0.02 0.01 0.009 0.005 0.6 0.3 3 0.11 0.4 0.3 0.2 1.1 0.025 0.015 0.010 0.003 0.7 0.7 4 0.09 0.4 0.2 0.1 1.9 0.03 0.02 0.005 0.005 0.6 0.5 5 0.08 0.2 0.4 0.2 1.7 0.02 0.01 0.009 0.003 0.6 0.5 6 0.11 0.4 0.3 0.1 1.2 0.035 0.015 0.008 0.005 0.7 0.3 7 0.10 0.4 0.3 0.2 1.8 0.02 0.01 0.01 0.005 0.7 0.7 8 0.080 0.2 0.4 0.2 1.4 0.015 0.025 0.02 0.004 0.6 0.5

[0053] Table 2. Hot rolling and cold rolling processes for steel in the examples.

[0054]

[0055] Table 3 Examples of Steel Annealing and Aluminizing Silicon Plating Processes

[0056]

[0057] Table 4 Mechanical properties and microstructure of the steels in the examples

[0058]

[0059]

[0060] 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 600 MPa, yield strength of 400–550 MPa, elongation of over 20%, and hole expansion value of over 90% are prepared. The microstructure of the steel plate is ferrite + bainite + martensite + pearlite, with ferrite content of 35%–61%, bainite content of 20%–35%, pearlite content of 6%–12%, and martensite content of 12%–25% by area percentage, thus achieving good strength, plasticity, and formability of the steel plate.

[0061] 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 600MPa grade hot-dip aluminized silicon high-perforation multiphase steel, comprising a substrate and a coating, characterized in that, The chemical composition of the substrate, by weight percentage, includes the following components: C: 0.08%~0.12%, Si: 1.0%~2.0%, Mn: 0.2%~0.4%, Cr: 0.1%~0.4%, Mo: 0.05%~0.2%, P≤0.02%, S≤0.005%, Ti: 0.01%~0.25%, Nb: 0.01%~0.03%, with the balance being Fe and unavoidable impurities; the steel plate has a tensile strength ≥600MPa, a yield strength 400~550MPa, an elongation >20%, and a hole expansion value ≥90%; the microstructure of the steel plate is ferrite + bainite + martensite + pearlite, with ferrite content of 35%~61%, bainite content of 20%~35%, pearlite content of 6%~12%, and martensite content between 12%~25% by area percentage.

2. The 600MPa grade hot-dip aluminized silicon high-expansion multiphase steel according to claim 1, characterized in that, The substrate contains Mn+Cr = 0.5%~0.8%. 。 3. The 600MPa grade hot-dip aluminized silicon high-expansion multiphase steel according to claim 1, characterized in that, The substrate has a Mo / Cr ratio of 1 / 3 to 2 / 3. 。 4. The 600MPa 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%.

5. A method for manufacturing 600MPa grade hot-dip aluminized silicon high-expansion multiphase steel according to any one of claims 1 to 4, comprising smelting, hot rolling, pickling, cold rolling, continuous annealing with aluminized silicon, and finishing, characterized in that, Hot rolling The heating temperature is between 1200 and 1250℃, and the holding time is ≥100 min; the initial rolling temperature is between 1100 and 1150℃, and the final rolling temperature is above 900℃; the coiling temperature is between 660 and 700℃, and the thickness of the hot-rolled coil is between 2.5 and 4.0 mm. cold rolling The cold rolling reduction rate is 40%~60%; Continuous annealing aluminum silicon plating ① The heating isothermal temperature is 760~820℃, 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; ② Aluminized silicon plating: After slow cooling, the strip steel enters the aluminum-silicon plating bath to obtain a coating on the steel plate surface. The temperature of the aluminum-silicon plating bath is 650~700℃, and the aluminum-silicon plating time is 3~10 s. ③ After aluminum-silicon plating, cool to 360~420℃ 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.

6. The method for manufacturing a 600MPa grade hot-dip aluminized silicon high-expansion multiphase steel according to claim 5, 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%.

7. The method for manufacturing a 600MPa grade hot-dip aluminized silicon high-expansion multiphase steel according to claim 5, characterized in that, The aluminum-silicon liquid plating solution used on the steel plate has the following composition: Al content 88~93%, Si content 7~12%.

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