1000MPa-grade alloyed galvanized complex-phase steel with ultrahigh reaming performance for automobile and preparation method of 1,000 MPa-grade alloyed galvanized complex-phase steel

Abstract

The invention belongs to the field of metal materials, and provides 1000MPa-grade alloyed galvanized multiphase steel with ultrahigh hole expansion performance for automobiles and a preparation method thereof, and the steel comprises the following components in percentage by weight: 0.12 to 0.19 percent of C, 1.50 to 2.50 percent of Mn, 0.30 to 0.7 percent of Si, 0.0002 to 0.0030 percent of B, 0.020 to 0.050 percent of Ti, 0.01 to 0.020 percent of Nb, less than or equal to 0.030 percent of P, less than or equal to 0.002 percent of N, less than or equal to 0.005 percent of S and the balance of Fe and inevitable impurities. The preparation method comprises the steps of smelting, hot rolling, acid pickling, cold rolling, continuous annealing galvanization and finishing, a quenching partition technology is adopted to enable the steel plate to mainly take tempered martensite and bainite phases, the hardness difference between soft-phase ferrite and hard-phase martensite is reduced, a smoother hardness gradient is manufactured, and the steel plate has the excellent forming performance while having the higher hole expansion rate.

Description

Technical Field

[0001] This invention belongs to the field of metallic materials, and particularly relates to 1000MPa grade alloyed galvanized ultra-high hole expansion performance multiphase steel for automobiles and its preparation method, which is especially suitable for roll forming of automotive safety and structural parts. Background Technology

[0002] While the continuous development of the automotive industry has brought convenience to people, it has also generated problems such as traffic congestion and environmental pollution, with CO2 emissions becoming a focus of attention for countries worldwide. In response, the automotive industry has proposed lightweighting as a crucial measure. A 10% reduction in vehicle weight can save 3% to 7% on fuel; a 100kg reduction in weight reduces fuel consumption by 0.6L / 100km. As the most important manufacturing material for automobiles, high-strength steel is the best way to achieve lightweighting, leading to the development of advanced high-strength steel for automobiles.

[0003] Advanced high-strength steel has now reached its third generation. Complex-phase (CP) steel is defined as high-strength steel containing two or more microstructure phases. More narrowly, it often refers to high-strength steel that incorporates phases such as bainite into the microstructure of dual-phase steel (ferrite and martensite). Compared to dual-phase steel, the introduction of bainite reduces the hardness difference between the phases, which is beneficial for improving the expansion rate and elongation flange performance. Therefore, CP steel is commonly used in the manufacture of automotive parts requiring high expansion performance, such as chassis, base beams, and seat rails.

[0004] The patent document "A 980MPa Grade Tempered Martensitic High-Expansion Steel and Its Manufacturing Method" (CN114107788B) discloses a high-expansion steel with the following chemical composition by weight percentage: C: 0.06%~0.10%, Si: 0.8%~2.0%, Mn: 1.5%~2.0%, P≤0.02%, S≤0.003%, Al: 0.02%~0.08%, N≤0.0004%, Mo: 0.1%~0.5%, Ti: 0.01%~0.05%, O≤0.0030%, with the remainder being Fe and other unavoidable impurities. Using the aforementioned chemical composition, through smelting, casting, hot rolling, cooling, bell-type annealing, and pickling, a high-expansion steel for automotive applications is obtained with a yield strength ≥800MPa, tensile strength ≥980MPa, elongation (transverse A50 ≥10%), impact toughness, and expansion performance (expansion rate ≥40%). The microstructure of this steel is tempered martensite, not multiphase steel, and the invention does not involve alloyed zinc-plated surfaces.

[0005] The patent document "1050MPa Grade Tempered Martensitic High Strength High-Expansion Steel and its Manufacturing Method" (CN116065094A) discloses the following chemical composition by weight percentage for high-expansion steel: C: 0.04%~0.1%, Si: 0.3%~1.0%, Mn: 1.3%~2.0%, Ti: 0.05%~0.1%, Mo: 0.1%~0.2%, Nb: 0.01%~0.05%, Ca: 0.005%~0.006%, Cr: 0%~0.5%, P≤0.005%, S≤0.005%, N≤0.003%, with the balance being Fe and other unavoidable impurities, and must simultaneously satisfy: 0.45≤Ti / Mo≤0.55. This chemical composition is processed through smelting, casting, homogenization, hot rolling, and tempering to produce high-strength, high-hole-expansion steel with a yield strength ≥980MPa, tensile strength ≥1050MPa, and hole expansion rate ≥40%. The high-hole-expansion steel in this patent contains the precious metal Mo, and the microstructure of the steel plate consists of tempered martensite and nano-carbides. It is a hot-rolled steel plate, not involving cold rolling or alloyed galvanizing.

[0006] Currently, the porosity of 1000MPa grade CP steel is typically 40%, and its plasticity is usually around 7%. Therefore, it cannot meet the forming and manufacturing requirements of automotive parts that demand both high plasticity and porosity. Furthermore, with the development of integrated automotive body structural components, traditional CP steel is even less able to meet the demands. Summary of the Invention

[0007] The purpose of this invention is to overcome the above-mentioned problems and deficiencies and provide a 1000MPa grade alloyed galvanized multiphase steel with ultra-high hole expansion performance for automobiles, which has both good elongation and ultra-high hole expansion rate, and its preparation method.

[0008] The objective of this invention is achieved as follows:

[0009] A 1000MPa grade alloyed galvanized ultra-high hole-expanding performance multiphase steel for automobiles has the following composition by weight percentage: C: 0.12-0.19%, Mn: 1.50%-2.50%, Si: 0.30%-0.7%, B: 0.0002%-0.0030%, Ti: 0.020%-0.050%, Nb: 0.01-0.020%, P: ≦0.030%, N≦0.002%, S≦0.005%, with the balance being Fe and unavoidable impurities.

[0010] The mass percentage of iron in the multiphase steel coating is 9.5% to 11.5%.

[0011] The multiphase steel comprises critical zone ferrite, oriented epiphytic ferrite, bainite, tempered martensite, retained austenite, and secondary martensite; by volume percentage as follows: critical zone ferrite 10%–20%, oriented epiphytic ferrite ≤12%, bainite 18%–35%, tempered martensite 15%–30%, retained austenite 7%–12%, and secondary martensite 8%–15%.

[0012] The multiphase steel plate has a tensile strength ≥1000MPa, a yield strength of 800~920MPa, an elongation after fracture ≥11%, and a hole expansion rate ≥60%.

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

[0014] Carbon (C) is an essential element in steel and the most effective strengthening element, existing as interstitial atoms to provide solid solution strengthening. Simultaneously, C combines with Ti in the steel to form carbides, which pin grain boundaries during rolling and heat treatment, thereby refining the grains and achieving a fine-grained, strong, and toughened effect. C is a strong austenite stabilizer and plays a crucial role in the retained austenite in this invention. However, excessive C easily forms cementite, significantly reducing the steel's plasticity and severely affecting its weldability. Therefore, this invention requires a C content of 0.12–0.19%.

[0015] Mn: Mn is the most important alloying element in this invention, mainly playing a role in solid solution strengthening. Mn is also an important austenite stabilizer, which can lower the critical quenching temperature of steel, delay the formation of austenite into pearlite, and contribute to obtaining the desired microstructure. Simultaneously, Mn can lower the onset temperature of martensitic transformation, stabilize austenite, and ensure the subsequent acquisition of retained austenite, thus contributing to the improvement of the experimental steel's plasticity. Excessive Mn addition easily leads to segregation at grain boundaries, causing the formation of banded structures, which severely affects the mechanical properties and hole-expanding properties of the steel plate. Therefore, this invention requires an Mn content of 1.50%–2.50%.

[0016] Si: Si exists in steel as substitutional atoms, effectively strengthening the ferrite matrix and reducing the hardness difference between ferrite and other phases, thus improving hole-expanding performance. Simultaneously, Si is an effective carbide suppressor, effectively inhibiting carbide precipitation during heat treatment, ensuring sufficient carbon in the austenite for greater stability, and ultimately obtaining retained austenite at room temperature. However, excessive Si addition is detrimental to achieving high surface quality in the coating. Therefore, this invention requires a Si content of 0.30%–0.7%.

[0017] Boron (B): Boron can effectively improve the hardenability of steel. It readily segregates at the original austenite grain boundaries, reducing the interfacial energy of these boundaries and inhibiting ferrite nucleation. This results in medium-to-low temperature phase transformation products, which is beneficial for obtaining the ideal microstructure of high-hole-expansion steel. However, excessive addition of boron can lead to the formation of borate carbides at the grain boundaries. Borate carbides can promote ferrite nucleation, which in turn reduces hardenability. Therefore, this invention requires a boron content of 0.0002% to 0.0030%.

[0018] Ti: Ti is an element that produces strong precipitation strengthening and good grain refinement in steel. This element can form stable compounds with N and C elements in steel, effectively pinning grain boundaries and refining the grain size of the steel plate, thus improving its mechanical properties. Simultaneously, Ti eliminates excess N elements at the sites of B element segregation in the experimental steel, effectively ensuring the non-equilibrium segregation of B elements to improve hardenability. Therefore, this invention requires a Ti content of 0.020%–0.050%.

[0019] Nb: In steel, Nb mainly exists as solid solution atoms or precipitates. Both forms strengthen the matrix phase and pin or drag the original grain boundaries, inhibiting grain growth during recovery and recrystallization, thus refining the grains. Its grain refining effect is more significant than that of Ti, resulting in increased strength and toughness of the steel sheet. Simultaneously, grain refinement leads to a more uniform microstructure, which is beneficial for improving hole-expanding performance. However, excessive Nb addition raises the recrystallization temperature, making production difficult and increasing the complexity of hot and cold rolling. Therefore, this invention requires an Nb content of 0.010–0.020%.

[0020] P: P is a harmful element in steel, easily causing grain boundary segregation and reducing the performance of the steel plate. Therefore, this invention requires P content to be ≤0.030%.

[0021] N: N is an impurity element in steel, and the lower its content, the better. Therefore, this invention requires that the N content be ≤0.002%.

[0022] S: S is an impurity element in steel, which readily reacts with Mn to form MnS, deteriorating the performance of the steel plate. Therefore, its content should be as low as possible. Thus, this invention requires an S content of ≤0.005%.

[0023] The second technical solution of the present invention is to provide a method for preparing 1000MPa grade alloyed galvanized ultra-high hole expansion performance multiphase steel for automobiles, including smelting, hot rolling, pickling, cold rolling, continuous annealing galvanizing, and finishing.

[0024] Smelting: The alloy composition within the above range is obtained by smelting in a converter and refining in an RH refining furnace, and then cast into a billet.

[0025] Hot-rolled:

[0026] ① The heating temperature is between 1225 and 1270℃, and the furnace time is ≥120 minutes. This limited temperature and time ensures uniform diffusion of all elements, eliminates element segregation in the cast billet, and makes the steel plate microstructure uniform. Simultaneously, within this temperature range, the nitrides formed by Ti and N elements in the steel refine the grains. Excessive heating temperature will cause excessive growth of the original austenite grains, while insufficient heating will reduce the solid solution of alloying elements such as Ti, thereby reducing the amount of carbide precipitation during subsequent cooling.

[0027] ② The roughing temperature is between 1100 and 1180℃, the finishing rolling temperature is between 1030 and 1090℃, the final rolling temperature is ≥890℃, and the thickness of the hot-rolled plate is 2.5 to 3.5 mm. This ensures that the rolling range of the steel plate is above the recrystallization temperature, so that recrystallization can occur fully and the grains can be refined.

[0028] ③ Post-rolling cooling is divided into two stages. The first stage cooling temperature is 700-780℃, and the second stage cooling rate is >70℃ / s. After the first stage of rolling, the steel plate is cooled to 700-780℃, and then the cooling rate is controlled to >70℃ / s in the second stage. This not only ensures sufficient precipitation of C and N compounds containing Ti or Nb, but also makes the precipitated phases appear in a dispersed precipitation state, which can maximize the effect of grain refinement. At the same time, the larger cooling rate in the second stage (>70℃ / s) can avoid the growth and aggregation of precipitated phases and inhibit the growth of ferrite grains, laying the foundation for the final uniformity of microstructure, which is beneficial to the improvement of hole expansion performance and strength.

[0029] ④ The coiling temperature should be between 545 and 630°C to avoid the formation of bainite at lower temperatures, which would increase the difficulty of cold rolling.

[0030] Pickling: Removes iron oxides from the surface of hot-rolled steel sheets to ensure the surface quality of cold-rolled sheets.

[0031] Cold rolling: The cold rolling reduction rate is controlled between 40% and 60% to ensure that the experimental steel obtains a high dislocation density, large deformation energy storage, and a good cold-rolled microstructure. This allows the steel plate to obtain a fine microstructure and dispersed precipitates during subsequent heat treatment, thereby ensuring the strength and hole expansion performance of the steel plate. However, an excessively high rolling reduction rate will increase the load on the cold rolling mill and may not guarantee the achievement of the target thickness.

[0032] Continuous annealing:

[0033] ① During the pre-oxidation stage, the dew point temperature is controlled at -20 to -5℃, the pre-oxidation temperature is 670 to 760℃, the annealing isothermal temperature is 885 to 925℃, and the annealing time is 30 to 100 seconds. Controlling the dew point and limiting the pre-oxidation temperature in the furnace ensures that the steel plate undergoes a certain degree of pre-oxidation, causing surface oxidation and preparing for subsequent alloying and galvanizing. This limited annealing temperature and time ensures sufficient recovery and recrystallization of the cold-rolled structure. Simultaneously, the annealing isothermal temperature of 885 to 925℃ results in a higher content of austenite and 10 to 20% critical zone ferrite, preparing for the final microstructure of the steel plate. The relatively low hardness of the critical zone ferrite in this stage, compared to the hardness of tempered martensite and bainite, is detrimental to achieving a high porosity.

[0034] ② The slow cooling rate is 2-4℃ / s, and the slow cooling temperature is 705-780℃. During the slow cooling stage, austenite mainly transforms into oriented epiphytic ferrite. Due to the slow diffusion rate of substitutional atoms such as Mn, the hardness of this part of ferrite is greater than that of the critical region ferrite. Moreover, the content of this part of ferrite is controlled by the slow cooling temperature. Therefore, this limited slow cooling temperature can ensure that the content of oriented epiphytic ferrite generated in the steel plate is ≤12%.

[0035] ③ After slow cooling, the steel is rapidly cooled to a temperature of 335–380℃ at a rate >28℃ / s: This rapid cooling rate ensures that the steel plate avoids transformations such as bainite and pearlite during this stage, only undergoing martensitic transformation. Simultaneously, the rapid cooling temperature of 335–380℃ determines that the martensite content produced at this stage is 25–35%, corresponding to tempered martensite in the final microstructure. The hardness of tempered martensite is not significantly different from that of bainite, ensuring a small hardness difference between the phases in the microstructure, which is crucial for achieving a high porosity.

[0036] ④ Alloying galvanizing: After rapid cooling, the steel plate is heated to 445-480℃ at a heating rate of 8-20℃ / s. The isothermal adjustment period before galvanizing is 15-30s. Then, it enters the zinc pot for galvanizing, with the zinc bath temperature at 460-470℃ and the galvanizing time at 1-3s. Subsequently, it enters the alloying furnace for the alloying process, with the alloying temperature at 475-495℃ and the alloying time at 10-25s. Finally, the steel plate is cooled to room temperature at a rate of >10℃ / s. Before entering the zinc pot, the steel plate temperature is adjusted to 445–480℃. This serves two purposes: firstly, it prevents fluctuations in the molten zinc due to temperature differences upon entering the zinc pot; secondly, this temperature causes the supercooled austenite to undergo bainitic transformation, while the martensite softens during tempering, resulting in carbon distribution into the supercooled austenite, thus stabilizing it and laying the foundation for the retention of room-temperature residual austenite. The limited temperature and time for alloying further ensure that the supercooled austenite in the steel plate undergoes bainitic transformation and martensitic tempering, yielding 18–35% bainite and 15–30% tempered martensite. Simultaneously, it induces zinc-iron interdiffusion in the coating, resulting in an iron content of 9.5–11.5%, meeting the requirements for pulverization performance. Excessive temperature or duration during the pre-galvanizing conditioning and alloying stages of the galvanizing process can cause martensite to precipitate as carbides, affecting carbon distribution and hindering the acquisition of retained austenite. Conversely, excessively low temperatures prevent effective martensitic tempering in the steel, affecting bainite formation and the final content of retained austenite. Furthermore, both excessively high and low alloying temperatures cannot guarantee the iron content of the coating. Finally, the steel plate is cooled to room temperature at a rate >10℃ / s. During this stage, some unstable supercooled austenite within the steel plate undergoes a martensitic transformation to form secondary martensite (8–15%), enhancing the steel plate's strength. The remaining austenite is retained at room temperature to form 7–12% retained austenite, which exhibits the TRIP effect during steel plate processing, increasing strength and elongation.

[0037] ⑤ After alloying and galvanizing, the steel sheet enters the finishing machine for sheet shape adjustment, and the finishing elongation is controlled at 0.2%~0.8%.

[0038] The final microstructure of the steel plate of this invention consists of 10–20% critical zone ferrite + ≤12% oriented epiphytic ferrite + 18–35% bainite + 15–30% tempered martensite + 7–12% retained austenite + 8–15% secondary martensite. The surface of the steel plate is coated with an alloyed zinc coating with an average iron content of 9.5–11.5%. Through the above innovative composition and process design, the tensile strength is ≥1000 MPa, the yield strength is 800–920 MPa, the elongation after fracture is ≥11%, and the porosity is ≥60%.

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

[0040] (1) The present invention uses C-Mn-Si as the main component, combined with a small amount of Ti and Nb, and does not add precious metal elements such as Cr and Mo. Therefore, it has a cost advantage over traditional CP steel.

[0041] (2) The present invention uses a quenching and partitioning process to make the steel plate mainly composed of tempered martensite and bainite phases, reducing the hardness difference between soft ferrite and hard martensite, creating a gentler hardness gradient, so that the steel plate has a higher hole expansion rate while also having excellent forming performance.

[0042] (3) While ensuring the realization of the alloying process, this invention achieves the excellent mechanical properties of CP steel with ultra-high hole expansion ratio, while ensuring that the Fe content of the coating meets the actual application requirements. Attached Figure Description

[0043] Figure 1 This is a metallographic image of the microstructure of the steel in Example 1 of the present invention. Detailed Implementation

[0044] The present invention will be further illustrated below through examples.

[0045] According to the component ratio of the technical solution, the embodiments of the present invention carry out smelting, continuous casting, rolling, continuous annealing and leveling.

[0046] Hot-rolled:

[0047] ① Heating temperature 1225~1270℃, furnace time ≥120min;

[0048] ② The roughing temperature is 1100~1180℃, the finishing rolling temperature is 1030~1090℃, the final rolling temperature is ≥890℃, and the thickness of the hot-rolled plate is 2.5~3.5mm;

[0049] ③ The post-rolling cooling is divided into two stages. The first stage cooling temperature is 700-780℃, and the second stage cooling rate is >70℃ / s.

[0050] ④ Winding temperature: 545~630℃;

[0051] Cold rolling: Cold rolling reduction rate 40-60%;

[0052] Continuous annealing:

[0053] ① During the pre-oxidation stage, the dew point temperature is controlled at -20 to -5℃, the pre-oxidation temperature is 670 to 760℃, the annealing isothermal temperature is 885 to 925℃, and the annealing time is 30 to 100 seconds;

[0054] ② The slow cooling rate is 2-4℃ / s, and the slow cooling temperature is 705-780℃;

[0055] ③ After slow cooling, the temperature is reduced to 335-380℃ at a rapid cooling rate of >28℃ / s;

[0056] ④ Alloying galvanizing: After rapid cooling, the steel plate is heated to 445-480℃ at a heating rate of 8-20℃ / s. The isothermal adjustment period before galvanizing is 15-30s. Then, it enters the zinc pot for galvanizing, with the zinc bath temperature at 460-470℃ and the galvanizing time at 1-3s. Subsequently, it enters the alloying furnace for the alloying process, with the alloying temperature at 475-495℃ and the alloying time at 10-25s. Finally, the steel plate is cooled to room temperature at a rate of >10℃ / s.

[0057] Furthermore, after alloying and galvanizing, the steel sheet enters the finishing machine for sheet shape adjustment, and the finishing elongation is controlled at 0.2%~0.8%.

[0058] The composition of the steel in this embodiment is shown in Table 1. The main process parameters for rolling the steel in this embodiment are shown in Table 2. The main process parameters for annealing and galvanizing the steel in this embodiment are shown in Table 3. The microstructure (vol%) of the steel in this embodiment is shown in Table 4. The properties of the steel in this embodiment are shown in Table 5.

[0059] Table 1. Composition (wt%) of steel in embodiments of the present invention

[0060] Example C Mn Si B Ti Nb P N S 1 0.14 2.05 0.63 0.0012 0.034 0.012 0.005 0.0013 0.001 2 0.17 1.68 0.52 0.0024 0.028 0.019 0.012 0.0008 0.003 3 0.13 1.93 0.36 0.0008 0.042 0.013 0.006 0.0012 0.002 4 0.15 1.96 0.65 0.0006 0.050 0.015 0.004 0.0015 0.004 5 0.17 1.87 0.45 0.0004 0.037 0.014 0.007 0.0006 0.001 6 0.18 2.05 0.43 0.0014 0.044 0.013 0.013 0.0007 0.002 7 0.12 2.36 0.58 0.0020 0.035 0.017 0.0024 0.0008 0.004 8 0.15 2.41 0.35 0.0009 0.026 0.016 0.003 0.0014 0.005

[0061] Table 2 Main process parameters for steel rolling in the embodiments of the present invention

[0062] Example Heating temperature / ℃ Insulation time / min Rough rolling temperature / ℃ Finishing rolling start temperature / ℃ Final rolling temperature / ℃ First stage cooling temperature / ℃ Second stage cooling rate / ℃ / s Winding temperature / ℃ Hot-rolled plate thickness / mm Cold rolling reduction rate / % 1 1258 130 1127 1038 912 718 85 621 2.9 46 2 1234 145 1132 1086 895 773 91 586 3.1 52 3 1227 128 1108 1054 906 736 74 567 3.2 51 4 1230 147 1174 1039 903 726 77 616 2.7 44 5 1248 152 1156 1048 910 753 81 593 3.0 58 6 1232 155 1163 1087 897 732 74 607 2.8 49 7 1267 138 1128 1063 902 741 76 598 3.4 59 8 1255 142 1114 1052 898 703 73 605 2.9 52

[0063] Table 3 Main heat treatment process parameters for steel annealing and galvanizing in embodiments of the present invention.

[0064] Example Dew point temperature / °C Pre-oxidation temperature / °C Isothermal temperature / ℃ Isothermal time / s Slow cooling rate / ℃ / s Slow cooling temperature / ℃ Rapid cooling rate / ℃ / s Rapid cooling temperature / ℃ Heating rate before galvanizing / ℃ / s Isothermal temperature before galvanizing / °C Isothermal time before galvanizing / s Zinc liquid temperature / ℃ Zinc plating time / s Alloying temperature / °C Alloying time / s rapid cooling rate / s 1 -18 747 898 63 2.8 711 34 341 12 457 21 467 2 486 17 13 2 -9 692 913 58 3.6 755 32 339 11 467 17 462 3 478 18 15 3 -14 704 920 67 3.2 743 35 350 9 452 16 468 1 492 20 21 4 -20 725 889 84 2.7 739 29 369 17 449 23 461 2 490 13 18 5 -17 683 894 39 2.2 778 35 337 14 461 28 463 2 486 15 23 6 -12 717 903 72 2.5 736 34 374 16 463 24 465 3 476 21 17 7 -19 732 912 46 3.8 726 33 361 19 475 19 462 2 483 16 14 8 -7 679 923 68 3.5 712 35 352 13 462 27 465 2 487 19 20

[0065] Table 4. Microstructure (vol%) of steel in the embodiments of the present invention

[0066] Example Critical region ferrite / % Oriented epiphytic ferrite / % Bainite / % Tempered martensite / % Residual austenite / % Secondary martensite / % 1 15 6 29 27 8 15 2 18 8 31 25 7 11 3 16 10 30 19 11 14 4 17 3 34 28 9 9 5 16 4 28 29 10 13 6 12 9 22 24 8 15 7 18 12 19 21 9 11 8 13 10 26 31 10 10

[0067] Table 5 Steel properties of embodiments of the present invention

[0068] Example Rp0.2 / MPa Rm / MPa A50 / % Hole expansion rate / % Iron content in coating / % 1 863 1082 12.1 64 11.4 2 837 1064 11.9 69 10.3 3 862 1077 12.3 67 11.2 4 893 1089 11.8 75 10.9 5 829 1038 12.5 65 10.7 6 846 1064 11.5 68 9.8 7 812 1039 12.4 74 10.3 8 912 1097 11.8 81 10.6

[0069] This invention, through low-cost composition design and innovative process design, produces alloyed galvanized high-hole-expansion multiphase steel plates mainly composed of tempered martensite, bainite, and retained austenite. These plates exhibit tensile strength ≥1000 MPa, yield strength of 800–920 MPa, elongation after fracture ≥11%, hole expansion rate ≥60%, and an iron content of 9.5%–11.5% by mass, achieving high elongation and ultra-high hole expansion rate for multiphase steel.

[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 1000MPa grade alloyed galvanized ultra-high hole-expanding performance multiphase steel for automobiles, characterized in that, The composition of this steel by weight percentage is as follows: C: 0.12-0.19%, Mn: 1.50%-2.50%, Si: 0.30%-0.7%, B: 0.0002%-0.0030%, Ti: 0.020%-0.050%, Nb: 0.01-0.020%, P: ≤0.030%, N ≤0.002%, S ≤0.005%, with the balance being Fe and unavoidable impurities.

2. The 1000MPa grade alloyed galvanized ultra-high hole-expanding performance multiphase steel for automobiles according to claim 1, characterized in that, The mass percentage of iron in the multiphase steel coating is 9.5% to 11.5%.

3. The 1000MPa grade alloyed galvanized ultra-high hole-expanding performance multiphase steel for automobiles according to claim 1, characterized in that, The multiphase steel comprises critical zone ferrite, oriented epiphytic ferrite, bainite, tempered martensite, retained austenite, and secondary martensite; by volume percentage as follows: critical zone ferrite 10%–20%, oriented epiphytic ferrite ≤12%, bainite 18%–35%, tempered martensite 15%–30%, retained austenite 7%–12%, and secondary martensite 8%–15%.

4. The 1000MPa grade alloyed galvanized ultra-high hole-expanding performance multiphase steel for automobiles according to claim 1, characterized in that, The multiphase steel plate has a tensile strength ≥1000MPa, a yield strength of 800~920MPa, an elongation after fracture ≥11%, and a hole expansion rate ≥60%.

5. A method for preparing a 1000MPa grade alloyed galvanized ultra-high hole-expanding performance multiphase steel for automobiles according to any one of claims 1-4, comprising smelting, hot rolling, pickling, cold rolling, continuous annealing galvanizing, and finishing; characterized in that: Hot-rolled: ① Heating temperature 1225~1270℃, furnace time ≥120min; ② The roughing temperature is 1100~1180℃, the finishing rolling temperature is 1030~1090℃, the final rolling temperature is ≥890℃, and the thickness of the hot-rolled plate is 2.5~3.5mm; ③ The post-rolling cooling is divided into two stages. The first stage cooling temperature is 700-780℃, and the second stage cooling rate is >70℃ / s. ④ Winding temperature: 545~630℃; Cold rolling: Cold rolling reduction rate 40%–60%; Continuous annealing: ① During the pre-oxidation stage, the dew point temperature is controlled at -20 to -5℃, the pre-oxidation temperature is 670 to 760℃, the annealing isothermal temperature is 885 to 925℃, and the annealing time is 30 to 100 seconds; ② The slow cooling rate is 2-4℃ / s, and the slow cooling temperature is 705-780℃; ③ After slow cooling, the temperature is reduced to 335-380℃ at a rapid cooling rate of >28℃ / s; ④ Alloying galvanizing: After rapid cooling, the steel plate is heated to 445-480℃ at a heating rate of 8-20℃ / s. The isothermal adjustment period before galvanizing is 15-30s. Then, it enters the zinc pot for galvanizing, with the zinc bath temperature at 460-470℃ and the galvanizing time at 1-3s. Subsequently, it enters the alloying furnace for the alloying process, with the alloying temperature at 475-495℃ and the alloying time at 10-25s. Finally, the steel plate is cooled to room temperature at a rate of >10℃ / s.

6. The method for preparing a 1000MPa grade alloyed galvanized ultra-high hole-expanding performance multiphase steel for automobiles according to claim 5, characterized in that: After alloying and galvanizing, the steel sheet enters the finishing machine for shape adjustment, and the finishing elongation is controlled at 0.2%~0.8%.

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