Low-cost 980MPa-grade high-plasticity automobile steel and preparation method thereof

Abstract

The invention discloses low-cost 980MPa-grade high-plasticity automobile steel and a preparation method thereof, and relates to the technical field of steel and iron material manufacturing, the low-cost 980MPa-grade high-plasticity automobile steel comprises, by weight, 0.24%-0.28% of C, 2.1%-2.5% of Mn, 0.8%-1.0% of Si, 0.7%-0.8% of Al, 0.09%-0.11% of V, 0.01%-0.02% of Ti and the balance Fe and impurities; the preparation method comprises the steps of smelting, casting, homogenization treatment, hot rolling, stress relief annealing, acid pickling, cold rolling, pre-quenching, two-phase region annealing and salt bath isothermal treatment. Through V-Ti multi-element microalloying design and in combination with pre-quenching treatment, the microstructure of the steel plate is adjusted into a composite structure of critical ferrite-bainite-retained austenite-martensite / austenite island-nano microalloy carbide; the prepared steel plate shows good strength and plasticity matching; and the cost is effectively controlled, meanwhile, good welding performance and coating performance are achieved, and light weight of an automobile is assisted.

Description

Technical Field

[0001] The present invention relates to the technical field of steel material manufacturing, and particularly to a low-cost 980 MPa grade high-plasticity automotive steel and a preparation method thereof. Background Art

[0002] The application of high-strength steel in automobile manufacturing can effectively reduce the body weight, reduce energy consumption, and improve the collision safety of vehicles at the same time; the 980 MPa grade high-strength automotive steel has a wide application prospect in key parts such as automobile structural parts and safety components due to its relatively high strength level. However, when traditional high-strength steel increases in strength, it is often accompanied by a decrease in plasticity, which brings difficulties to the processing and forming of the steel, and may affect its use performance under complex working conditions, reducing the reliability and durability of the vehicle.

[0003] The problems of the steel in the prior art include: (1) It is difficult to achieve a more ideal balance among strength, plasticity, and cost; for example, in the patent "A Preparation Method for Simultaneously Improving the Strength and Plasticity of Niobium Microalloyed TRIP Steel" with the publication number CN118441134A, although the simultaneous improvement of strength and plasticity is achieved, the tensile strength of this steel product is ~860 MPa, and in the automotive field, its strength level is insufficient, making it difficult to balance body weight reduction and structural safety; in addition, 0.79 wt.% of Ni element is added to this steel, increasing its manufacturing cost; in the patent "A High-Strength and High-Plasticity Product Cold-Rolled TRIP Steel and a Preparation Method Thereof" with the publication number CN106636925B, the tensile strength of this steel is 869 - 977 MPa, and the yield strength is 572 - 658 MPa, which also cannot meet the application scenario requirements of high strength and high load; in the patents "An 1180 MPa Grade Ultra-High Strength Low-Cost Cold-Rolled Quenching and Partitioning Steel and a Manufacturing Method Thereof" with the publication number CN110093564A and "A 900 MPa Grade High-Strength Steel with Excellent Plasticity and Toughness and a Heat Treatment Method for Increasing Its Retained Austenite Content" with the publication number CN115233092A, their plasticity is relatively low, and it is difficult to meet the diverse forming requirements when facing the processing of complex parts, showing obvious application limitations; (2) The processing difficulty is relatively large and the production efficiency is low. For example, in the patent "A Heat Treatment Method for Low-Carbon Microalloyed High-Strength and High-Plasticity Product Cold-Rolled TRIP980 Steel" with the publication number 109182923B, 0.025 - 0.045 wt.% of Nb element is added, which produces a dragging and strain-induced precipitation effect, resulting in an excessive increase in the strength of the hot-rolled coil after coiling, increasing the equipment load and processing difficulty during cold rolling and reducing the production efficiency; in the patent "A Preparation Method for Medium-Manganese Steel with Low Manganese Content" with the publication number 115181913B, the relatively high Mn content will cause non-uniform microstructure, which is not conducive to production and processing, and the relatively high C content will deteriorate the welding performance, restricting the popularization and use of this steel type in actual application scenarios that require welding processes. Summary of the Invention

[0004] The object of the present invention is to provide a steel for automobiles with excellent comprehensive performance, which can not only reduce the weight of the vehicle body, lower energy consumption and costs, but also ensure structural safety during collisions and is easy to process.

[0005] To solve the above technical problems, the present invention adopts the following technical solutions: A high-plasticity steel for automobiles with a low cost of 980 MPa grade. By weight percentage, the composition of the steel plate for automobiles includes C: 0.24 - 0.28%, Mn: 2.1 - 2.5%, Si: 0.8 - 1.0%, Al: 0.7 - 0.8%, V: 0.09 - 0.11%, Ti: 0.01 - 0.02%, and the balance is Fe and impurities.

[0006] Preferably, the microstructure of the steel plate mainly consists of critical ferrite, bainite, retained austenite, martensite / austenite islands, and nano microalloy carbides; and

[0007] The volume fraction of retained austenite accounts for 17 - 18%, and is uniformly distributed in the bainite matrix in the form of lath or film, and the microalloy carbides precipitate in the matrix at the nanoscale.

[0008] Preferably, the yield strength of the steel plate is not less than 720 MPa, and / or the tensile strength is not less than 980 MPa, and / or the elongation is not less than 28%, and / or the product of strength and plasticity is not less than 30 GPa·%.

[0009] The present invention also discloses a preparation method of a high-plasticity steel for automobiles with a low cost of 980 MPa grade, including the following steps:

[0010] S1. Prepare the steel billet raw materials, and the composition of the steel billet raw materials by weight percentage is as follows:

[0011] C: 0.24 - 0.28%, Mn: 2.1 - 2.5%, Si: 0.8 - 1.0%, Al: 0.7 - 0.8%, V: 0.09 - 0.11%, Ti: 0.01 - 0.02%, and the balance is Fe and impurities;

[0012] S2. Melt the steel billet raw materials and perform ingot casting;

[0013] S3. Hot-roll the ingot to obtain a hot-rolled steel plate;

[0014] S4. Pickle the hot-rolled steel plate to remove the surface scale;

[0015] S5. Cold-roll the pickled hot-rolled steel plate in multiple passes to obtain cold-rolled sheets;

[0016] S6. Perform pre-quenching treatment on the cold-rolled sheets;

[0017] S7. Anneal the pre-quenched sheet in the two-phase region.

[0018] S8. Quench the sheet annealed in the two-phase region in a salt bath to 370 - 380 °C, hold for 290 - 310 s, and then air-cool to room temperature.

[0019] Preferably, in the pre-quenching treatment, heat the sheet to 880 - 900 °C, hold for 240 - 300 s, and then water-quench to room temperature.

[0020] Preferably, in the two-phase region annealing, heat the pre-quenched sheet to 770 - 790 °C and hold for 230 - 250 s.

[0021] Preferably, obtain ingots in portions of 20 Kg in weight and 20 - 30 mm in thickness, heat the ingots to 1100 - 1150 °C and hold for at least 1 hour.

[0022] Preferably, hot-roll the ingots into steel plates with a thickness of 4.5 mm, where 1100 °C ≤ initial rolling temperature ≤ 1150 °C, 950 °C ≤ final rolling temperature, and then air-cool to room temperature.

[0023] Preferably, heat the steel plates to 580 - 600 °C for stress relief annealing, and perform pickling after air-cooling.

[0024] Preferably, the thickness of the sheet after cold rolling is 1.1 - 1.3 mm.

[0025] Beneficial effects: In the steel plate of the present invention, the addition of 0.24 - 0.28 wt.% of C element can improve the stability of austenite without seriously negatively affecting the welding performance of the steel; 2.1 - 2.5 wt.% of Mn element can expand the austenite region range, increase the retained austenite content, and improve the mechanical properties of the steel. The addition of 0.8 - 1.0 wt.% of Si element can inhibit the precipitation of carbides during low-temperature heating. The presence of carbides will consume C element in the steel and affect the volume fraction of retained austenite; at the same time, a lower Si content can improve the coating and plating properties of the steel plate and enhance the surface quality; the addition of 0.7 - 0.8 wt.% of Al element can reduce weight and save energy while also inhibiting the precipitation of cementite and improving the stability of retained austenite; the addition of V-Ti multi-component microalloying elements can refine the grains and produce a precipitation strengthening effect, increasing the yield strength and tensile strength of the steel.

[0026] Among them, the volume fraction of retained austenite is also increased to 17 - 18%, so that the elongation of the 980 MPa grade high-plasticity automotive steel is increased to 28 - 29%; the microalloyed carbides precipitate in the matrix at the nanoscale; with its grain refinement characteristics and precipitation strengthening effect, it can not only reduce the weight of the body, reduce energy consumption and costs, but also ensure the structural safety during collision.

[0027] In the preparation method of the present invention, through the introduction of the pre-quenching process, before heat treatment, the internal structure of the cold-rolled sheet is completely transformed into lath martensite, then heated to 770 - 790 °C and held for 230 - 250 s for critical austenitization, quenched to 370 - 380 °C and held in a salt bath for 290 - 310 s; part of the critical austenite undergoes bainite transformation to obtain low-carbon bainite with high strength and fine laths; among them, the occurrence of bainite transformation causes the carbon element in it to diffuse and enrich into austenite, improving the stability of austenite and being able to retain more retained austenite; compared with equiaxed and massive retained austenite, the mechanical stability of film-like retained austenite is higher; after it is applied to the preparation of automotive parts, in case of vehicle collision, a large amount of film-like retained austenite will continuously undergo martensite transformation, inducing plastic growth and absorbing collision energy, thereby improving the automotive collision safety performance;

[0028] In addition, both the pre-quenching treatment and the microalloying technology are beneficial to the refinement of the microstructure; the refinement of ferrite can disperse the premature stress / strain concentration of retained austenite and delay the generation of cracks, while the refinement of retained austenite can improve the stability of retained austenite, give full play to the TRIP effect, and improve the coordinated deformation ability of each phase, achieving the effect of simultaneously improving strength and plasticity. Description of the Drawings

[0029] The drawings are used to provide a further understanding of the present invention and constitute a part of the specification. Together with the embodiments of the present invention, they are used to explain the present invention and do not constitute a limitation to the present invention.

[0030] In the drawings:

[0031] Figure 1 are the microstructural diagrams observed by the scanning electron microscope and transmission electron microscope of the present invention. Detailed Embodiments

[0032] The embodiments of the present invention will be described below with reference to the drawings in the embodiments of the present invention. The terms used in the embodiments of the present invention are only used to explain the specific embodiments of the present invention and are not intended to limit the present invention. The embodiments of the present application will be described below with reference to the drawings.

[0033] A kind of low-cost 980 MPa grade high-plasticity steel for automobiles, by weight percentage, the composition of the steel plate for automobiles includes C: 0.24 - 0.28%, Mn: 2.1 - 2.5%, Si: 0.8 - 1.0%, Al: 0.7 - 0.8%, V: 0.09 - 0.11%, Ti: 0.01 - 0.02%, and the balance is Fe and impurities;

[0034] According to the above composition ranges, three experimental steels were prepared; high-purity raw materials were configured into 20 Kg of raw materials by mass percentage, melted in a vacuum induction furnace, and then subjected to ingot casting with a thickness of 20 - 30 mm. The preparation method is as follows:

[0035] S1: Heat the ingot to 1100 - 1150 °C and hold for at least 1 hour for homogenization treatment;

[0036] S2: Hot-roll the ingot obtained in S1 into a hot-rolled steel plate with a thickness of 4.5 mm, where 1100 °C ≤ initial rolling temperature ≤ 1150 °C, 950 °C ≤ final rolling temperature, and then air-cool to room temperature;

[0037] S3: Heat the hot-rolled steel plate obtained in S2 to 580 - 600 °C for stress relief annealing to reduce hardness, and perform pickling after air-cooling to remove the surface oxide scale;

[0038] S4: Perform multi-pass cold rolling on the pickled hot-rolled steel plate obtained in S3 to obtain a cold-rolled sheet with a thickness of 1.1 - 1.3 mm, taking 1.2 mm as an example;

[0039] S5: Heat the cold-rolled sheet obtained in S4 to 880 - 900 °C, hold for 240 - 300 s, and then water-quench to room temperature to complete the pre-quenching treatment;

[0040] The prior intervention of the pre-quenching treatment heats the cold-rolled sheet to the fully austenitizing temperature range and then rapidly cools it, causing the structure to transform into lath martensite; the martensite structure has a high density of dislocations and a specific crystal orientation, creating unique conditions for subsequent processing; when entering the critical annealing stage in the two-phase region, based on the high dislocation density of martensite, a large number of nucleation sites can be provided at the phase interface, significantly increasing the number of austenite nucleations and remarkably enhancing the austenite content; meanwhile, the crystal defects and internal stresses introduced during the pre-quenching process will change the chemical and physical environment of austenite, optimize its composition distribution, promote the growth of austenite along the lath boundaries of martensite, showing a lath or film-like distribution, enhancing the stability of austenite, so that during the subsequent cooling process, more austenite can be retained to room temperature as retained austenite;

[0041] S6: Heat the cold-rolled sheet after completing the pre-quenching in S5 to 770 - 790 °C and hold for 230 - 250 s for two-phase region annealing; promote the formation of critical austenite;

[0042] S7: Quench the cold-rolled sheet after completing the two-phase region annealing in S6 in a salt bath to 370 - 380 °C, hold for 290 - 310 s, and then air-cool to room temperature; obtain a mixed structure mainly composed of critical ferrite, bainite, retained austenite, and martensite / austenite islands;

[0043] By adjusting the holding temperature in the two-phase region, the volume fraction and stability of the critical austenite can be controlled. A relatively low critical annealing temperature is not conducive to the nucleation and growth of reverse-transformed austenite. Although a too high temperature can increase the content of reverse-transformed austenite, the full partitioning of alloying elements will lead to a decrease in its thermal stability. During the subsequent bainite transformation process, the critical austenite with low thermal stability will transform into bainite, reducing the retained austenite content in the steel. As a hard phase, the large formation of bainite can endow the material with high strength, but it can also lead to a decrease in plasticity, making the material show a certain brittle tendency. To balance the strength and plasticity of the material, the formation amount and morphology of bainite need to be precisely controlled in the process. In the present invention, by setting the bainite transformation temperature to 370 - 380 °C, it is beneficial to the formation of lower bainite. The fine and dispersed carbides in lower bainite can better maintain the plasticity of the material while increasing the strength. This benefits from its organizational structure that can effectively hinder the movement of dislocations and does not easily produce serious stress concentration like the coarse upper bainite structure. In addition, the precipitation of microalloy carbides not only brings about a fine grain strengthening effect, but its precipitation strengthening effect also further improves the strength of the steel. By comprehensively regulating these process parameters and element additions, it is possible to effectively improve the plasticity of the material while obtaining high strength, significantly improving the comprehensive mechanical properties of the material.

[0044] Based on the above composition range and preparation method, Examples 1 - 3 are provided. The actual chemical compositions of the steel plates in the examples are shown in Table 1. Four groups of samples are cut from the prepared steel plates and processed at different heat treatment temperatures within the heat treatment temperature range. The heat treatment process parameters of all samples are shown in Table 2. Among them, the No. 1 specimen in Examples 1 - 3 only undergoes traditional TRIP treatment to study the influence of pre-quenching on the mechanical properties of high-strength steel by comparison. Standard tensile specimens are machined according to national standards, and then room temperature tensile tests are carried out. The mechanical properties of the steel plates in the examples are shown in Table 3.

[0045] In addition, Comparative Examples 1 - 6 in Tables 1 - 3 are the compositions, process parameters and corresponding mechanical properties of high-strength automotive steels prepared by existing processes for comparison.

[0046] Table 1 shows the chemical composition of the steel plate in wt%:

[0047]

[0048]

[0049] Table 2 shows the heat treatment process parameters of the steel plate:

[0050]

[0051]

[0052] Wherein: T Q : quenching temperature; T q : quenching time; T L : annealing temperature in two-phase region; t L : annealing time in two-phase region; T T : bainite isothermal transformation temperature; t T : bainite isothermal time.

[0053] Table 3 shows the mechanical properties of the steel plate:

[0054]

[0055]

[0056]

[0057] Wherein: Rp0.2: yield strength; Rm: tensile strength; A: elongation; PSE: product of strength and plasticity;

[0058] From the data in Table 1, Table 2 and Table 3, it can be seen that for the high-strength automotive steels produced by the preparation methods of the low-cost 980 MPa grade high-plasticity automotive steels in Example 1, Example 2 and Example 3 of the present invention, the yield strength is not less than 720 MPa, the tensile strength is not less than 980 MPa, the elongation is not less than 28%, and the product of strength and plasticity is not less than 30 GPa·%; comparing the No. 2-4 specimens with the No. 1 specimen in each example, it can be seen that the introduction of the pre-quenching process, although reducing the strength of the steel to a certain extent, increases its plasticity by 40-45%, and the product of strength and plasticity also increases from 24-25 GPa·% to 29-30 GPa·%.

[0059] Compared with Comparative Example 1 and Comparative Examples 4-6, the steel plate products of the present invention exhibit a higher strength level, and can effectively reduce the body weight while ensuring the safety and reliability of the automotive structure; although Comparative Examples 2-3 have a comparable strength level to the steel plate products of the present invention, their plasticity is relatively poor, and defects such as cracking are extremely likely to occur during complex forming processes, greatly limiting their application in fields with high requirements for material formability such as automotive manufacturing.

[0060] In terms of composition and process, the treatment temperature of some specimens in Comparative Example 1 is similar to that of the present invention. However, the addition of precious metal Ni increases the manufacturing cost. Compared with Comparative Examples 2-3 with equivalent heat treatment temperature and time, the product of the present invention adds Al element in the composition design, which can effectively reduce the overall density of the material and has significant meaning for the lightweight of automobile bodies. Comparing with Comparative Examples 4-6 which also add Al element, their higher treatment temperature not only increases energy consumption but also puts forward higher requirements for the high-temperature resistance performance of the equipment, increasing the equipment purchase and maintenance costs. To sum up, the present invention adopts a more reasonable treatment temperature, effectively controls the production cost while ensuring the high performance and lightweight of the product, and occupies an obvious advantage in the market competition.

[0061] As Figure 1 shown, through scanning electron microscopy and transmission electron microscopy observation and analysis. B: bainite; RA: retained austenite; IF: critical ferrite; MC carbide: MC-type microalloy carbide; M / A: martensite / austenite island. The matrix of the high-strength steel of the present invention is composed of a mixed structure mainly composed of critical ferrite, bainite, retained austenite and martensite / austenite island, and nanoscale MC-type carbides are dispersed. Through the preparation method of the present invention, most of the retained austenite can be promoted to be distributed in the form of thin films at the bainite phase boundary. Compared with massive retained austenite, such thin-film retained austenite has better mechanical stability and plays a key role in the process of the material being stressed. When the material bears an external load, the retained austenite continuously undergoes strain-induced martensite transformation, which can efficiently dissipate the energy input from the outside, effectively inhibit the initiation and propagation of cracks, and thus significantly improve the toughness and work hardening ability of the material. At the same time, in view of the fact that the retained austenite itself has certain plasticity, its existence can coordinate the deformation between the surrounding hard phases (such as bainite), avoid excessive stress concentration, and enable the entire mixed structure to maintain structural integrity when bearing a large deformation.

[0062] The embodiments of the present invention have been described in detail above in conjunction with the accompanying drawings. However, the present invention is not limited to the above embodiments. For those of ordinary skill in the art in this technical field, after learning the content recorded in the present invention, without departing from the principle of the present invention, several equivalent transformations and substitutions can still be made, and these equivalent transformations and substitutions should also be regarded as belonging to the protection scope of the present invention.

Claims

1. A low-cost 980 MPa grade high-plasticity automotive steel, characterized in that: By weight percentage, the composition of the steel sheet for automobiles includes C: 0.24 - 0.28%, Mn: 2.1 - 2.5%, Si: 0.8 - 1.0%, Al: 0.7 - 0.8%, V: 0.09 - 0.11%, Ti: 0.01 - 0.02%, with the balance being Fe and impurities.

2. The steel for automobile according to claim 1, characterized in that: The microstructure of the steel sheet mainly consists of critical ferrite, bainite, retained austenite, martensite / austenite islands, and nano micro-alloy carbides; and The volume fraction of retained austenite accounts for 17 - 18%, and it is uniformly distributed in the bainite matrix in the form of laths or thin films, and the micro-alloy carbides precipitate in the matrix at the nanoscale.

3. The automotive steel according to claim 1 or 2, characterized in that: The yield strength of the steel sheet is not less than 720 MPa and / or the tensile strength is not less than 980 MPa, and / or the elongation is not less than 28%, and / or the product of strength and plasticity is not less than 30 GPa·%.

4. A preparation method of a low-cost 980 MPa high-plasticity automotive steel, characterized in that, It includes the following steps: S1. Prepare the steel billet raw materials, and the composition of the steel billet raw materials by weight percentage is as follows: C: 0.24 - 0.28%, Mn: 2.1 - 2.5%, Si: 0.8 - 1.0%, Al: 0.7 - 0.8%, V: 0.09 - 0.11%, Ti: 0.01 - 0.02%, with the balance being Fe and impurities; S2. Melt the steel billet raw materials and carry out ingot casting; S3. Hot-roll the ingot to obtain a hot-rolled steel sheet; S4. Pickle the hot-rolled steel sheet to remove the surface scale; S5. Cold-roll the pickled hot-rolled steel sheet in multiple passes to obtain a cold-rolled sheet; S6. Carry out pre-quenching treatment on the cold-rolled sheet; S7. Heat the pre-quenched sheet for two-phase region annealing; S8. Quench the sheet after two-phase region annealing in a salt bath to 370 - 380 °C, hold for 290 - 310 s, and then air-cool to room temperature.

5. The preparation method according to claim 4, characterized in that: During the pre-quenching treatment, heat the sheet to 880 - 900 °C, hold for 240 - 300 s, and then water-quench to room temperature.

6. The preparation method according to claim 5, characterized in that: During the two-phase region annealing, heat the pre-quenched sheet to 770 - 790 °C and hold for 230 - 250 s.

7. The preparation method according to any one of claims 4 to 6, characterized in that: Obtain an ingot with a weight of 20 Kg and a thickness of 20 - 30 mm as one portion, heat the ingot to 1100 - 1150 °C, and hold for at least 1 hour.

8. The preparation method according to any one of claims 4 to 6, characterized in that: Hot-roll the ingot into a steel sheet with a thickness of 4.5 mm, where 1100 °C ≤ initial rolling temperature ≤ 1150 °C, 950 °C ≤ final rolling temperature, and then air-cool to room temperature.

9. The preparation method according to any one of claims 4-6, characterized in that: Heat the steel sheet to 580 - 600 °C for stress relief annealing, and carry out pickling after air-cooling.

10. The preparation method according to any one of claims 4-6, characterized in that: The thickness of the cold-rolled sheet is 1.1 - 1.3 mm after cold rolling.

Images