Low-cost 980mpa grade high ductility automotive steel and method of making same

Abstract

The application discloses a low-cost 980MPa-grade high-plasticity automobile steel and a preparation method thereof, and relates to the technical field of steel material manufacturing. According to the weight percentage, the composition of the steel plate comprises 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 of Fe and impurities. The preparation method comprises the following steps: smelting, casting, homogenization treatment, hot rolling, stress relief annealing, pickling, cold rolling, pre-quenching, two-phase zone annealing, salt bath isothermal treatment, and the like. Through V-Ti multi-element micro-alloying design and in combination with pre-quenching treatment, the microstructure of the steel plate is adjusted to a composite structure of critical ferrite-bainite-residual austenite-martensite / austenite island-nanometer micro-alloy carbide. The prepared steel plate exhibits good strength-plasticity matching. While effectively controlling the cost, the steel plate is endowed with good weldability and plating property, and helps the light weight of automobiles.

Description

Technical Field

[0001] This invention relates to the field of steel material manufacturing technology, specifically to a low-cost 980MPa grade high-plasticity automotive steel and its preparation method. Background Technology

[0002] The application of high-strength steel in automobile manufacturing can effectively reduce vehicle weight, decrease energy consumption, and improve vehicle collision safety. 980MPa grade high-strength automotive steel, due to its high strength, has broad application prospects in critical components such as automotive structural parts and safety devices. However, while increasing strength, traditional high-strength steel often comes at the cost of reduced plasticity. This makes processing and forming difficult and may affect its performance under complex working conditions, reducing vehicle reliability and durability.

[0003] The problems with steel in the prior art include: (1) it is difficult to achieve a more ideal balance between strength and plasticity and cost; for example, the patent with publication number CN118441134A, "A method for preparing niobium microalloyed TRIP steel with simultaneous improvement of strength and plasticity", although it achieves simultaneous improvement of strength and plasticity, the tensile strength of the steel product is ~860MPa, which is insufficient in the automotive field, making it difficult to balance the weight reduction of the car body and structural safety; in addition, 0.79wt.% of Ni element is added to the steel, which increases its manufacturing cost; the patent with publication number CN106636925B, "A high strength and plasticity cold-rolled TRIP steel and its preparation method", has a tensile strength of 869-977MPa and a yield strength of 572-658MPa, which also cannot meet the application requirements of high strength and high load-bearing capacity; the patent with publication number CN110093564A, "A 1180MPa grade ultra-high strength low cost cold-rolled quenched fractional steel and its manufacturing method", and the patent with publication number CN118441134A, "A method for preparing niobium microalloyed TRIP steel with simultaneous improvement of strength and plasticity", and the patent with publication number CN118441134A, "A method for preparing niobium microalloyed TRIP steel with simultaneous improvement of strength and plasticity", has a tensile strength of 869-977MPa and a yield strength of 572-658MPa, which also cannot meet the application requirements of high strength and high load-bearing capacity; the patent with publication number CN110093564A, "A method for preparing 1180MPa grade ultra-high strength low cost cold-rolled quenched fractional steel and its manufacturing method", and the patent with publication number CN118441134A, "A method for preparing niobium microalloyed TRIP The patent CN115233092A, "High-strength steel with excellent plasticity and toughness of 900MPa grade and heat treatment method to improve its residual austenite content", has low plasticity and is difficult to meet the diverse forming requirements when facing complex parts processing, and has obvious application limitations; (2) The processing difficulty is large and the production efficiency is low. For example, the patent with publication number 109182923B, "A heat treatment method for low carbon microalloyed high strength plasticity cold-rolled TRIP980 steel", adds 0.025-0.045wt.% Nb element, which produces drag and strain-induced precipitation effect, resulting in excessive strength increase after hot-rolled plate is coiled, which increases the equipment load and processing difficulty in the cold rolling process and reduces production efficiency; the patent with publication number 115181913B, "A preparation method of low manganese content medium manganese steel", has a high Mn content, which will cause uneven microstructure, which is not conducive to production and processing. The high C content will deteriorate the welding performance, which limits the promotion and use of this steel in actual application scenarios that require welding process. Summary of the Invention

[0004] The purpose of this invention is to provide an automotive steel with excellent comprehensive performance that can reduce vehicle weight, energy consumption and cost, ensure structural safety during collisions, and is easy to process.

[0005] To solve the above-mentioned technical problems, the present invention adopts the following technical solution: a low-cost 980MPa grade high-plasticity automotive steel, wherein the composition of the automotive steel sheet by weight percentage 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.

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

[0007] The volume fraction of retained austenite is 17-18%, and it is uniformly distributed in the bainitic matrix in the form of laths or thin films. Microalloyed carbides are precipitated in the matrix at the nanoscale.

[0008] Preferably, the steel plate has a yield strength of not less than 720 MPa, and / or a tensile strength of not less than 980 MPa, and / or an elongation of not less than 28%, and / or a strength-ductility product of not less than 30 GPa·s.

[0009] This invention also discloses a low-cost method for preparing 980MPa grade high-ductility automotive steel, comprising the following steps:

[0010] S1. Prepare steel billet raw materials, wherein the steel billet raw materials have the following composition by weight percentage:

[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%, balance Fe and impurities;

[0012] S2. The steel billet raw material is melted and then cast in mold;

[0013] S3. Hot-rolled ingots are used to obtain hot-rolled steel plates;

[0014] S4. Pickling hot-rolled steel plate to remove surface oxide scale;

[0015] S5. Hot-rolled steel sheets after multiple cold rolling and pickling processes are used to obtain cold-rolled sheet metal.

[0016] S6. Pre-quench the cold-rolled sheet material.

[0017] S7. After pre-quenching, the plate is annealed in the two-phase region.

[0018] S8. After annealing in the two-phase region, the plate is quenched in a salt bath to 370–380°C, held for 290–310 seconds, and then air-cooled to room temperature.

[0019] Preferably, in the pre-quenching treatment, the plate is heated to 880-900°C, held at that temperature for 240-300 seconds, and then water-quenched to room temperature.

[0020] Preferably, in the two-phase annealing process, the pre-quenched plate is heated to 770-790°C and held for 230-250 seconds.

[0021] Preferably, ingots are obtained in portions of 20 kg in weight and 20-30 mm in thickness, and the ingots are heated to 1100-1150°C and held at that temperature for at least 1 hour.

[0022] Preferably, the ingot is hot-rolled into a steel plate with a thickness of 4.5 mm, wherein 1100℃≤initial rolling temperature≤1150℃, 950℃≤final rolling temperature, and then air-cooled to room temperature.

[0023] Preferably, the steel plate is heated to 580-600°C for stress-relieving annealing, and then air-cooled before pickling.

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

[0025] Beneficial effects: In the steel plate of this invention, the addition of 0.24–0.28 wt.% C element improves the stability of austenite without seriously negatively affecting the weldability of the steel; 2.1–2.5 wt.% Mn element expands the austenite region, increases the content of retained austenite, and improves the mechanical properties of the steel; the addition of 0.8–1.0 wt.% Si element can inhibit the precipitation of carbides at low temperatures, and the presence of carbides consumes C element in the steel, affecting the volume fraction of retained austenite; at the same time, the lower Si content can improve the coatability of the steel plate and enhance the surface quality; the addition of 0.7–0.8 wt.% Al element reduces weight and saves 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, thereby improving 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 980MPa grade high plasticity automotive steel is increased to 28-29%; microalloyed carbides are precipitated in the matrix at the nanoscale; with its grain-refining characteristics and precipitation strengthening effect, it can not only reduce the weight of the vehicle body, reduce energy consumption and cost, but also ensure structural safety during collision.

[0027] In the preparation method of this invention, the introduction of a pre-quenching process causes the internal structure of the cold-rolled sheet to completely transform into lath martensite before heat treatment. It is then heated to 770-790℃ and held for 230-250s for critical austenitization, followed by quenching to 370-380℃ and salt bath holding for 290-310s. This causes some of the critical austenite to undergo bainitic transformation, resulting in high-strength, fine-grained, low-carbon bainite. The bainitic transformation causes carbon to diffuse and accumulate in the austenite, improving its stability and retaining more residual austenite. Compared to equiaxed and blocky residual austenite, film-like residual austenite exhibits higher mechanical stability. When applied to the manufacture of automotive parts, in the event of a vehicle collision, a large amount of film-like residual austenite will continuously undergo martensitic transformation, inducing plastic growth, absorbing collision energy, and thus improving the vehicle's collision safety performance.

[0028] In addition, both pre-quenching treatment and microalloying technology are beneficial to the refinement of microstructure; ferrite refinement can disperse the premature stress / strain concentration of retained austenite and delay crack initiation, while retained austenite refinement can improve the stability of retained austenite, give full play to the TRIP response, enhance the coordinated deformation ability of each phase, and achieve the effect of simultaneous improvement of strength and plasticity. Attached Figure Description

[0029] The accompanying drawings are provided to further illustrate the invention and form part of the specification. They are used together with the embodiments of the invention to explain the invention and do not constitute a limitation thereof.

[0030] In the attached diagram:

[0031] Figure 1 These are microscopic tissue images observed by scanning electron microscope and transmission electron microscope of this invention. Detailed Implementation

[0032] The embodiments of the present invention will now be described with reference to the accompanying drawings. The terminology used in the embodiments section is for illustrative purposes only and is not intended to limit the scope of the invention. The embodiments of this application will now be described with reference to the accompanying drawings.

[0033] A low-cost, 980MPa grade, high-plasticity automotive steel, wherein the composition of the automotive steel sheet by weight percentage 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;

[0034] Based on the above composition range, three experimental steels were prepared. High-purity raw materials were prepared in batches of 20 kg by mass percentage, melted in a vacuum induction furnace, and then cast in molds to a thickness of 20–30 mm. The preparation method included the following:

[0035] S1: Heat the ingot to 1100-1150℃ and hold for at least 1 hour to homogenize it;

[0036] S2: The ingot obtained from S1 is hot-rolled into a hot-rolled steel plate with a thickness of 4.5 mm, wherein 1100℃≤initial rolling temperature≤1150℃, 950℃≤final rolling temperature, and then air-cooled to room temperature;

[0037] S3: The hot-rolled steel plate obtained in S2 is heated to 580-600℃ for stress-relief annealing to reduce hardness. After air cooling, it is pickled to remove the surface oxide scale.

[0038] S4: The hot-rolled steel sheet obtained from S3 after pickling is subjected to multiple cold rolling passes to obtain a cold-rolled sheet with a thickness of 1.1-1.3mm, taking 1.2mm as an example;

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

[0040] The pre-quenching process heats the cold-rolled sheet to the full austenitization temperature range, followed by rapid cooling, transforming the microstructure into lath martensite. Martensite, with its high dislocation density and specific crystal orientation, creates unique conditions for subsequent processing. When entering the critical annealing stage in the two-phase region, the high dislocation density of martensite provides numerous nucleation sites at the phase interface, significantly increasing the number of austenite nuclei and thus boosting the austenite content. Simultaneously, the crystal defects and internal stresses introduced during pre-quenching alter the chemical and physical environment of austenite, optimizing its compositional distribution and promoting austenite growth along the martensite lath interface, resulting in a lath or film-like distribution. This enhances austenite stability, allowing more austenite to be retained at room temperature as residual austenite during subsequent cooling.

[0041] S6: Heat the cold-rolled sheet material that has been pre-quenched by S5 to 770-790℃ and hold for 230-250s to perform two-phase annealing; promote the formation of critical austenite.

[0042] S7: The cold-rolled sheet that has undergone two-phase annealing in S6 is quenched in a salt bath to 370-380℃, held for 290-310s and then air-cooled to room temperature; to obtain a mixed microstructure 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 critical austenite can be controlled. A lower critical annealing temperature is not conducive to the nucleation and growth of reverse-transformed austenite, while an excessively high temperature, although it can increase the content of reverse-transformed austenite, will lead to a decrease in its thermal stability due to the full distribution of alloying elements. During the subsequent bainitic transformation, the critical austenite with low thermal stability transforms into bainite, reducing the residual austenite content in the steel. While the abundant formation of bainite, as a hard phase, can impart high strength to the material, it can also lead to a decrease in plasticity, causing the material to exhibit a certain tendency towards brittleness. To balance the strength and plasticity of the material, the amount and morphology of bainite formation need to be precisely controlled during the process. In this invention, by setting the bainitic transformation temperature to 370-380℃, the formation of lower bainite is facilitated. The finely dispersed carbides in lower bainite can better maintain the plasticity of the material while improving strength. This is because its microstructure can effectively hinder dislocation movement, while unlike the coarse upper bainite structure, it is not prone to severe stress concentration. In addition, the precipitation of microalloyed carbides not only brings a fine-grain strengthening effect, but its precipitation strengthening effect also further enhances the strength of the steel. By comprehensively controlling these process parameters and element additions, it is possible to obtain high strength while effectively improving the plasticity of the material, thus significantly improving the overall mechanical properties of the material.

[0044] Based on the above composition range and preparation method, Examples 1-3 are provided. The actual chemical composition of the steel plates in the examples is shown in Table 1. Four groups of samples were cut from the prepared steel plates and treated 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, sample No. 1 in Examples 1-3 was only subjected to traditional TRIP treatment to compare and study the effect of pre-quenching on the mechanical properties of high-strength steel. Standard tensile samples were processed according to national standards and then subjected to room temperature tensile tests. 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 show the composition, process parameters and corresponding mechanical properties of high-strength automotive steel prepared by existing processes for comparison.

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

[0047]

[0048]

[0049] Table 2 shows the heat treatment process parameters for steel plates:

[0050]

[0051]

[0052] Wherein: T Q Quenching temperature; T q Quenching time; T L : Two-phase annealing temperature; t L : Two-phase annealing time; T T Isothermal transformation temperature of bainite; t T Bainite isothermal time.

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

[0054]

[0055]

[0056]

[0057] Where: Rp0.2: yield strength; Rm: tensile strength; A: elongation; PSE: strength-ductility product;

[0058] As can be seen from the data in Tables 1, 2, and 3, the high-strength automotive steel produced by the low-cost 980MPa-grade high-plasticity automotive steel preparation method of Examples 1, 2, and 3 of this invention has a yield strength of not less than 720MPa, a tensile strength of not less than 980MPa, an elongation of not less than 28%, and a strength-ductility product of not less than 30GPa·%, exhibiting excellent strength-ductility matching. Comparing samples 2-4 of each example with sample 1, it can be seen that although the introduction of the pre-quenching process reduces the strength of the steel to a certain extent, it increases its plasticity by 40-45%, and the strength-ductility product also increases from 24-25GPa·% to 29-30GPa·%.

[0059] Compared with Comparative Examples 1 and 4-6, the steel plate product of the present invention exhibits a higher strength level, which can effectively reduce the weight of the vehicle body while ensuring the safety and reliability of the vehicle structure. Although Comparative Examples 2-3 have a similar strength level to the steel plate product of the present invention, their plasticity is relatively poor. When faced with complex forming processes, they are prone to defects such as cracking, which greatly limits their application in fields such as automobile manufacturing where the formability of materials is highly demanding.

[0060] Regarding composition and process, the processing temperature of some samples in Comparative Example 1 is similar to that of this invention. However, the addition of the precious metal Ni increases manufacturing costs. Compared with Comparative Examples 2-3, which have similar heat treatment temperatures and times, the product of this invention incorporates Al in its composition design, which effectively reduces the overall density of the material and has significant implications for lightweighting automobile bodies. In contrast, Comparative Examples 4-6, which also contain Al, have higher processing temperatures that not only increase energy consumption but also place higher demands on the high-temperature resistance of the equipment, increasing equipment purchase and maintenance costs. In summary, this invention employs a more reasonable processing temperature, effectively controlling production costs while ensuring high performance and lightweighting of the product, giving it a clear advantage in market competition.

[0061] like Figure 1 As shown, the results were observed and analyzed using scanning electron microscopy and transmission electron microscopy. B: Bainite; RA: Retained austenite; IF: Critical ferrite; MC Carbide: MC-type microalloyed carbides; M / A: martensite / austenite islands. The high-strength steel matrix of this invention is composed of a mixed structure mainly consisting of critical ferrite, bainite, retained austenite, and martensite / austenite islands, with nanoscale MC-type carbides dispersed throughout. Through the preparation method of this invention, the retained austenite is mostly distributed in a thin film form at the bainite phase boundary. Compared with blocky retained austenite, this thin film-like retained austenite has better mechanical stability and plays a key role in the material stress process. When the material is subjected to external loads, the retained austenite continuously undergoes strain-induced martensitic phase transformation. This process can efficiently dissipate externally input energy, 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, given that the retained austenite itself has a certain degree of plasticity, its presence 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 subjected to large deformations.

[0062] The embodiments of the present invention have been described in detail above with reference to the accompanying drawings. However, the present invention is not limited to the above embodiments. For those skilled in the art, after learning the contents described in the present invention, several equivalent changes and substitutions can be made without departing from the principle of the present invention. These equivalent changes and substitutions should also be considered to fall within the protection scope of the present invention.

Claims

1. A low-cost, high-ductility automotive steel of 980MPa grade, characterized in that: By weight percentage, the composition of automotive steel sheets 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; The microstructure of the steel plate consists of critical ferrite, bainite, retained austenite, martensite / austenite islands, and nano-alloyed carbides; and The volume fraction of retained austenite is 17-18%, and it is uniformly distributed in the bainitic matrix in the form of laths or thin films. Microalloyed carbides are precipitated in the matrix at the nanoscale. The steel plate has a yield strength of not less than 720 MPa, a tensile strength of not less than 980 MPa, an elongation of not less than 28%, and a strength-ductility product of not less than 30 GPa∙.

2. A method for preparing a low-cost, high-ductility automotive steel of 980MPa grade, characterized in that, Includes the following steps: S1. Prepare steel billet raw materials, wherein the steel billet raw materials have the following composition by weight percentage: 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%, balance Fe and impurities; S2. Melt the steel billet raw material and then cast it in a mold; S3. Hot-rolled ingots are used to obtain hot-rolled steel plates; S4. Pickling hot-rolled steel plate to remove surface oxide scale; S5. Hot-rolled steel sheets after multiple cold rolling and pickling processes are used to obtain cold-rolled sheet metal. S6. Pre-quench the cold-rolled sheet material. S7. After pre-quenching, the plate is annealed in the two-phase region. S8. After the two-phase region annealing, the plate is quenched in a salt bath to 370-380℃, held for 290-310s and then air-cooled to room temperature. In the pre-quenching process, the plate is heated to 880-900℃, held for 240-300 seconds, and then water-quenched to room temperature. In two-phase annealing, the pre-quenched plate is heated to 770-790℃ and held for 230-250 seconds.

3. The preparation method according to claim 2, characterized in that: Ingots are obtained in portions of 20 kg each with a thickness of 20-30 mm. The ingots are heated to 1100-1150°C and held at that temperature for at least 1 hour.

4. The preparation method according to claim 2, characterized in that: The ingot is hot-rolled into a steel plate with a thickness of 4.5 mm, wherein the initial rolling temperature is 1100℃≤1150℃ and the final rolling temperature is 950℃≤the final rolling temperature, and then air-cooled to room temperature.

5. The preparation method according to claim 2, characterized in that: The steel plate is heated to 580-600℃ for stress-relieving annealing, and then air-cooled and pickled.

6. The preparation method according to claim 2, characterized in that: The thickness of the sheet obtained by cold rolling is 1.1-1.3 mm.

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