980mpa grade dual phase steel and method of making

Abstract

The application relates to a 980MPa-grade dual-phase steel and a preparation method, and belongs to the technical field of steel plate manufacturing. The chemical composition of the dual-phase steel is as follows in terms of mass fraction: C: 0.11%-0.13%, Si: 0.30%-0.40%, Mn: 2.40%-2.60%, Cr: 0.50%-0.70%, Alt: 0.02%-0.05%, Nb: 0.01%-0.03%, Ti: 0.02%-0.03%, P<=0.015%, S<=0.007%, and the rest is Fe and inevitable impurities; the microstructure of the dual-phase steel comprises ferrite and martensite. By optimizing the C and Mn ratio and replacing Mo with Cr, combined with Nb and Ti micro-alloying to realize grain refinement and band structure regulation, relying on precise composition control, hot rolling controlled cooling and low-temperature coiling process, and matching the continuous galvanizing technology, the dual-phase steel with high yield strength, good elongation, enhanced work hardening effect and high hole expansion performance is developed, and the precise stamping forming demand of automobile complex structural parts is met.

Description

Technical Field

[0001] This application relates to the field of steel plate manufacturing technology, and in particular to a 980MPa grade duplex steel and its preparation method. Background Technology

[0002] In recent years, the downward pressure on the automobile consumer market has intensified, and steel companies are simultaneously facing the dual challenges of fluctuating energy prices and tightening environmental policies, leading to an imbalance in the industry's supply and demand structure and increased risk of price volatility. Against this backdrop, developing high-strength steel products that combine low cost and excellent formability has become a key path to improving corporate profitability and market competitiveness. However, high-strength steel typically requires the addition of excessive alloying elements to expand the austenite phase region to meet strength requirements, and current reduction-weight alloying designs struggle to meet mechanical performance standards. In particular, the current mainstream continuously annealed duplex steels generally employ molybdenum alloying systems, and the high production costs of continuously annealed duplex steels significantly compress profit margins, resulting in a continuous weakening of product market competitiveness. Summary of the Invention

[0003] This application provides a 980MPa grade duplex steel and its preparation method to solve the following technical problem: how to achieve a balance between high hole expansion performance and low production cost in 980MPa grade duplex steel. In a first aspect, embodiments of this application provide a 980MPa grade duplex steel, the chemical composition of which, by mass fraction, is: C: 0.11%~0.13%, Si: 0.30%~0.40%, Mn: 2.40%~2.60%, Cr: 0.50%~0.70%, Alt: 0.02%~0.05%, Nb: 0.01%~0.03%, Ti: 0.02%~0.03%, P≤0.015%, S≤0.007%, with the remainder being Fe and unavoidable impurities; The microstructure of the dual-phase steel includes ferrite and martensite.

[0004] Optionally, the volume fraction of ferrite is 50% to 60%, and the volume fraction of martensite is 40% to 50%.

[0005] Optionally, the duplex steel meets at least one of the following properties: yield strength: 700MPa~850MPa, tensile strength ≥980MPa, elongation ≥8%.

[0006] Secondly, embodiments of this application provide a method for preparing the duplex steel described in the first aspect, the method comprising: A slab with the following chemical composition was obtained: C: 0.11%~0.13%, Si: 0.30%~0.40%, Mn: 2.40%~2.60%, Cr: 0.50%~0.70%, Alt: 0.02%~0.05%, Nb: 0.01%~0.03%, Ti: 0.02%~0.03%, P≤0.015%, S≤0.007%, with the remainder being Fe and unavoidable impurities; The slab is sequentially heated and precision rolled to obtain hot-rolled steel strip; The hot-rolled steel strip is sequentially cooled, coiled, and heat-treated to obtain duplex steel.

[0007] Optionally, the furnace exit temperature is 1240℃~1270℃, and the holding time is 160min~180min.

[0008] Optionally, the finishing rolling temperature is 860℃~900℃.

[0009] Optionally, the thickness of the hot-rolled steel strip is 2.5mm to 5.5mm.

[0010] Optionally, the cooling adopts ultra-fast medium-pressure cooling and front-end dense cooling, and the cooling rate is 25℃ / s~35℃ / s.

[0011] Optionally, the winding temperature is 530℃~570℃.

[0012] Optionally, the heating temperature of the heat treatment is 795℃~805℃, the heat soaking temperature of the heat treatment is 795℃~805℃, and the slow cooling temperature of the heat treatment is 640℃~660℃.

[0013] Optionally, the rapid cooling temperature of the heat treatment is 300℃~320℃, and the over-aging temperature of the heat treatment is 290℃~310℃.

[0014] Optionally, the belt speed of the heat treatment is 70m / min to 150m / min, and the finishing elongation of the heat treatment is 0.2% to 0.4%.

[0015] Optionally, the thickness of the duplex steel is ≤2.6mm.

[0016] The technical solutions provided in this application have the following advantages compared with the prior art: This application provides a 980MPa grade duplex steel. The chemical composition of the duplex steel, by mass fraction, is: C: 0.11%~0.13%, Si: 0.30%~0.40%, Mn: 2.40%~2.60%, Cr: 0.50%~0.70%, Alt: 0.02%~0.05%, Nb: 0.01%~0.03%, Ti: 0.02%~0.03%, P≤0.015%, S≤0.007%, with the remainder being Fe and unavoidable impurities. The microstructure of the duplex steel comprises ferrite and martensite. This application achieves a balance between high hole-expanding performance and low production cost for 980MPa grade duplex steel through low-cost alloy design combined with microstructure control. In terms of composition, C (0.11%~0.13%) and Mn (2.40%~2.60%) are the main components, supplemented with Cr (0.50%~0.70%) to improve hardenability, ensuring sufficient hard phase (martensite) can be formed under conventional cooling conditions. This avoids the use of expensive elements such as Mo and V, significantly reducing alloy costs. Trace amounts of Nb (0.01%~0.03%) and Ti (0.02%~0.03%) inhibit austenite grain growth by forming nanoscale carbonitrides, refining the final microstructure and improving material uniformity and formability. The low Si design (0.30%~0.40%) reduces the formation of iron oxide scale on the hot-rolled surface, improves edge quality, and reduces the risk of crack initiation during punching or hole expansion. Impurities such as P and S are controlled at extremely low levels (P≤0.015%, S≤0.007%), reducing grain boundary embrittlement and improving the cleanliness and toughness of the steel. The microstructure of the dual-phase steel should primarily consist of ferrite and martensite. By controlling the cooling path, the soft ferrite and hard martensite are rationally distributed. The plasticity of ferrite alleviates stress concentration in the martensitic region, thereby suppressing early crack propagation during the hole expansion process and achieving high hole expansion performance. Simultaneously, the combined effect of fine-grain strengthening and phase transformation strengthening ensures a tensile strength exceeding 980 MPa.

[0017] In summary, this solution, through economical component matching and microstructure optimization, balances strength, formability, and production cost without requiring complex processes, thus resolving the contradiction between the poor hole expansion properties and high cost of traditional high-strength steel. Attached Figure Description

[0018] The accompanying drawings, which are incorporated in and form part of this specification, illustrate embodiments consistent with this application and, together with the description, serve to explain the principles of this application.

[0019] To more clearly illustrate the technical solutions in the embodiments of this application or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, for those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0020] Figure 1 A flowchart illustrating a 980MPa grade duplex steel and its preparation method is provided for embodiments of this application. Figure 2 Tissue morphology image taken by the electron microscope equipment provided in Embodiment 4 of this application; Figure 3 The tissue morphology image is taken by the light microscope device provided in Embodiment 4 of this application. Detailed Implementation

[0021] To make the objectives, technical solutions, and advantages of the embodiments of this application clearer, the technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some, not all, of the embodiments of this application. All other embodiments obtained by those skilled in the art based on the embodiments of this application without creative effort are within the scope of protection of this application.

[0022] The range descriptions used herein, such as numerical ranges and proportional ranges, include all possible sub-ranges and single numerical values ​​within that range. For example, the range descriptions of "1 to 6" or "1~6" cover all sub-ranges between 1 and 6 (such as 1 to 3, 2 to 5, etc.) and single numbers (such as 1, 2, 3, 4, 5, 6). Unless otherwise specified, the terms "including" and "contains" used herein mean "including but not limited to"; relational terms such as "first" and "second" are used only to distinguish different entities or operations and do not imply an actual order or relationship. "And / or" indicates that multiple situations can exist individually or simultaneously. Expressions such as "at least one," "multiple," and "at least one" refer to any combination of the corresponding objects, including combinations of single or multiple objects. The proportional relationships mentioned herein, such as mass ratios and molar ratios, should be understood as the correspondence between the first and second terms of a proportional formula, according to the order of description. The raw materials, reagents, instruments, and equipment used herein can all be obtained through commercial purchase or prepared using existing methods.

[0023] In a first aspect, embodiments of this application provide a 980MPa grade duplex steel, the chemical composition of which, by mass fraction, is: C: 0.11%~0.13%, Si: 0.30%~0.40%, Mn: 2.40%~2.60%, Cr: 0.50%~0.70%, Alt: 0.02%~0.05%, Nb: 0.01%~0.03%, Ti: 0.02%~0.03%, P≤0.015%, S≤0.007%, with the remainder being Fe and unavoidable impurities; The positive effects of limiting the C mass fraction to 0.11%~0.13% include: synergistic optimization of the martensitic phase transformation driving force and ferrite ductility, significantly suppressing grain boundary embrittlement tendency while ensuring a tensile strength of 980MPa, and achieving a synergistic improvement in strength and porosity. For example, the C mass fraction can be 0.11%, 0.12%, 0.13%, etc.

[0024] The positive effects of limiting the Si mass fraction to 0.30%~0.40% include: enhancing the driving force for ferrite formation and improving phase transformation plasticity, while effectively balancing the requirements for surface oxidation inhibition and coating adhesion, thus achieving synergistic optimization of microstructure control and surface quality. For example, the Si mass fraction can be 0.30%, 0.32%, 0.34%, 0.36%, 0.38%, 0.40%, etc.

[0025] The positive effects of limiting the mass fraction of Mn to 2.40%~2.60% include: enhancing austenite stability and hardenability while precisely suppressing segregation band formation, thus simultaneously achieving a triple guarantee of increased strength, controlled banded structure, and weld compatibility. For example, the mass fraction of Mn can be 2.40%, 2.45%, 2.50%, 2.55%, 2.60%, etc.

[0026] The positive effects of limiting the Cr mass fraction to 0.50%~0.70% include: effectively replacing the hardenability function of Mo while precisely suppressing the formation of manganese segregation bands, achieving synergistic optimization of hole-expanding performance and weld compatibility in low-cost alloy systems. For example, the Cr mass fraction can be 0.50%, 0.55%, 0.60%, 0.65%, 0.70%, etc.

[0027] The positive effects of limiting the mass fraction of Alt to 0.02%~0.05% include: achieving efficient deoxidation and grain refinement while precisely suppressing the formation of coarse oxide inclusions, and simultaneously ensuring the synergistic optimization of steel cleanliness and hot-rolled surface quality. For example, the mass fraction of Alt can be 0.02%, 0.03%, 0.04%, 0.05%, etc.

[0028] The positive effects of limiting the Nb mass fraction to 0.01%~0.03% include: achieving strong grain boundary pinning and precipitation strengthening while precisely controlling recrystallization kinetics, thus simultaneously achieving a triple synergistic effect of grain refinement, strength enhancement, and pore-expanding performance optimization. For example, the Nb mass fraction can be 0.01%, 0.02%, 0.03%, etc.

[0029] The positive effects of limiting the Ti mass fraction to 0.02%~0.03% include: achieving efficient N element solidification and precipitate size control while precisely suppressing weld heat-affected zone coarsening, simultaneously achieving a triple guarantee of grain refinement, aging risk elimination, and weld compatibility. For example, the Ti mass fraction can be 0.02%, 0.03%, etc.

[0030] The positive effects of limiting the mass fraction of phosphorus (P) to ≤0.015% include: effectively suppressing grain boundary segregation and low-temperature embrittlement while ensuring the interfacial bonding strength of the coating, achieving a key synergy between improving material toughness and controlling surface quality. For example, the mass fraction of P can be 0.001%, 0.003%, 0.005%, 0.007%, 0.009%, 0.011%, 0.013%, 0.015%, etc.

[0031] The positive effects of limiting the mass fraction of sulfur (S) to ≤0.007% include: effectively preventing the segregation of low-melting-point sulfides and the risk of hot brittleness while ensuring the plasticity and rheological properties of the matrix, thus achieving key synergistic optimization of the material's hot working properties and cold forming ductility. For example, the mass fraction of S can be 0.001%, 0.003%, 0.005%, 0.007%, etc.

[0032] Fe is a matrix element, and the specific content / range of Fe can be obtained through the upper and lower limit formulas of the component, that is: The sum of the percentages of all components in a composition should equal 100%, and the content ranges of several components should meet the following conditions: the upper limit of a certain component + the lower limit of other components ≤ 100; the lower limit of a certain component + the upper limit of other components ≥ 100. Furthermore, the specific content of Fe is made up to 100% by the actual detected values ​​of the other chemical components mentioned above, together with any unlisted active elements and / or impurity elements, and Fe must constitute the absolute proportion as a matrix element.

[0033] The microstructure of the dual-phase steel includes ferrite and martensite.

[0034] In some embodiments, the volume fraction of ferrite is 50% to 60%, and the volume fraction of martensite is 40% to 50%.

[0035] The synergistic ratio of ferrite and martensite volume fractions constructs the core mechanical equilibrium framework of the dual-phase microstructure. The continuously distributed ferrite matrix provides fundamental plasticity assurance for the dual-phase steel, while the dispersed martensitic reinforcing phase endows it with high strength properties. Ultimately, this achieves precise optimization of the strength-ductility matching relationship, giving the dual-phase steel a comprehensive advantage in high strain forming capability and impact toughness. For example, the ferrite volume fraction can be 50%, 52%, 54%, 56%, 58%, 60%, etc.; the martensite volume fraction can be 40%, 42%, 44%, 46%, 48%, 50%, etc.

[0036] In some embodiments, the duplex steel satisfies at least one of the following properties: yield strength: 700MPa~850MPa, tensile strength ≥980MPa, elongation ≥8%.

[0037] Yield strength: The critical stress threshold at which a material begins to undergo significant plastic deformation. It characterizes the ability of a structural component to resist permanent deformation under service loads and directly determines the crashworthiness of automotive safety components. Tensile strength: The maximum engineering stress a material can withstand under static tensile conditions. It reflects the ultimate load-bearing capacity of a component and provides a high strength-to-weight ratio guarantee for lightweight design. Elongation: The plastic deformation capacity of a material before fracture. It quantifies the tolerance space for uniform deformation during stamping and forming, avoiding localized thinning and cracking in complex parts. For example, yield strength can be 700MPa, 750MPa, 800MPa, 850MPa, etc.; tensile strength can be 980MPa, 990MPa, 1000MPa, 1010MPa, 1020MPa, etc.; elongation can be 8%, 9%, 10%, etc.

[0038] Figure 1 A flowchart illustrating a 980MPa grade duplex steel and its preparation method, provided as an embodiment of this application.

[0039] Please see Figure 1 Secondly, this application provides a method for preparing the duplex steel described in the first aspect, the method comprising: S1. Obtain a slab with the following chemical composition: C: 0.11%~0.13%, Si: 0.30%~0.40%, Mn: 2.40%~2.60%, Cr: 0.50%~0.70%, Alt: 0.02%~0.05%, Nb: 0.01%~0.03%, Ti: 0.02%~0.03%, P≤0.015%, S≤0.007%, with the remainder being Fe and unavoidable impurities; S2. The slab is heated and precision rolled sequentially to obtain hot-rolled steel strip; S3. The hot-rolled steel strip is sequentially cooled, coiled, and heat-treated to obtain duplex steel.

[0040] In some embodiments, the furnace exit temperature is 1240℃~1270℃, and the holding time is 160min~180min.

[0041] The furnace exit temperature is between 1240℃ and 1270℃. Precise control of this temperature window ensures sufficient dissolution of niobium-titanium carbides while preventing abnormal coarsening of austenite grains, providing optimized initial microstructure conditions for dynamic recrystallization during hot rolling. For example, the furnace exit temperature can be 1240℃, 1250℃, 1260℃, or 1270℃. The holding time is between 160min and 180min to ensure sufficient austenitization of the slab core and homogenization of element diffusion, eliminating the tendency for banded microstructure inheritance caused by continuous casting segregation. For example, the holding time can be 160min, 165min, 170min, 175min, or 180min.

[0042] In some embodiments, the finishing rolling temperature is 860°C to 900°C.

[0043] The finishing rolling temperature is between 860℃ and 900℃ to fully stimulate dynamic recrystallization and refine austenite grains while precisely avoiding the rolling risks in the non-recrystallized region, thus providing uniform and fine-grained precursors for subsequent ultrafast cold phase transformation. For example, the finishing rolling temperature can be 860℃, 870℃, 880℃, 890℃, 900℃, etc.

[0044] In some embodiments, the thickness of the hot-rolled steel strip is 2.5 mm to 5.5 mm.

[0045] The thickness of hot-rolled steel strip is between 2.5mm and 5.5mm. This is to precisely meet the differentiated requirements of lightweighting and load-bearing capacity for automotive structural components while fully matching the belt speed control window of the continuous annealing production line, achieving a key synergy between process adaptability and product versatility. For example, the thickness of the steel strip can be 2.5mm, 3.0mm, 3.5mm, 4.0mm, 4.5mm, 5.0mm, 5.5mm, etc.

[0046] In some implementations, the cooling adopts an ultra-fast medium-pressure cooling mode and a front-end dense cooling mode, and the cooling rate is 25℃ / s to 35℃ / s.

[0047] The cooling process employs ultra-fast intermediate-pressure cooling and front-end intensive cooling. Ultra-fast intermediate-pressure cooling achieves high-penetration phase transformation suppression, while simultaneous front-end intensive cooling eliminates microstructure differences between the beginning and end of the strip, synergistically achieving precise control and performance homogenization of the bainite-dominated microstructure throughout the coil. The cooling rate is between 25℃ / s and 35℃ / s, synergistically matching the coiling temperature window to drive the efficient transformation of supercooled austenite to bainite, preventing the risk of bending cracks caused by abnormal martensite formation. For example, the cooling rate can be 25℃ / s, 27℃ / s, 29℃ / s, 31℃ / s, 33℃ / s, 35℃ / s, etc.

[0048] In some embodiments, the winding temperature is 530°C to 570°C.

[0049] The winding temperature is between 530℃ and 570℃ to fully activate the bainite-dominated phase transformation while blocking the abnormal formation of pearlite and martensite, thus providing an ideal initial microstructure matrix with high uniformity and low internal stress for the continuous annealing process. For example, the winding temperature can be 530℃, 540℃, 550℃, 560℃, 570℃, etc.

[0050] In some embodiments, the heating temperature of the heat treatment is 795°C to 805°C, the soaking temperature of the heat treatment is 795°C to 805°C, and the slow cooling temperature of the heat treatment is 640°C to 660°C.

[0051] The heating temperature and soaking temperature of the heat treatment are both between 795℃ and 805℃. Precise control of this temperature range ensures sufficient austenitization and drives uniform carbon diffusion while preventing grain coarsening, laying the foundation for compositional and nucleation homogeneity in the subsequent ferrite-martensite dual-phase structure. For example, the heating temperature can be 795℃, 797℃, 799℃, 801℃, 803℃, or 805℃; the soaking temperature can also be 795℃, 797℃, 799℃, 801℃, 803℃, or 805℃. The slow cooling temperature of the heat treatment is between 640℃ and 660℃. Precise control of the ferrite transformation termination point achieves a synergistic balance between soft phase ratio optimization and retained austenite carbon enrichment, ensuring compatibility between the martensitic transformation driving force and porosity expansion performance. For example, the slow cooling temperature can be 640℃, 645℃, 650℃, 655℃, or 660℃.

[0052] In some embodiments, the rapid cooling temperature of the heat treatment is 300°C to 320°C, and the over-aging temperature of the heat treatment is 290°C to 310°C.

[0053] The rapid cooling temperature of the heat treatment is between 300℃ and 320℃. Precise control of this temperature window fully triggers the explosive phase transformation of martensite while preventing premature precipitation of carbides, achieving synergistic regulation of the stability of high-strength martensite islands and retained austenite. For example, the rapid cooling temperature of the heat treatment can be 300℃, 305℃, 310℃, 315℃, 320℃, etc. The over-aging temperature of the heat treatment is between 290℃ and 310℃, with the aim of softening the martensite structure and reducing the hardness difference between martensite and ferrite. For example, the over-aging temperature of the heat treatment can be 290℃, 300℃, 310℃, etc.

[0054] In some embodiments, the belt speed of the heat treatment is 70 m / min to 150 m / min, and the finishing elongation of the heat treatment is 0.2% to 0.4%. The belt speed of the heat treatment is 70 m / min to 150 m / min.

[0055] The heat treatment belt speed is between 70 m / min and 150 m / min, dynamically matching the austenitization time window for strips of different thicknesses. This ensures sufficient microstructure transformation while optimizing production line efficiency, achieving a balance between process versatility and economy. For example, the heat treatment belt speed can be 70 m / min, 80 m / min, 90 m / min, 100 m / min, 110 m / min, 120 m / min, 130 m / min, 140 m / min, 150 m / min, etc. The finishing elongation of the heat treatment is between 0.2% and 0.4%, effectively eliminating the yield plateau phenomenon while precisely suppressing work hardening overload, achieving key synergistic optimization of stamping dimensional accuracy control and forming stability. For example, the finishing elongation of the heat treatment can be 0.2%, 0.3%, 0.4%, etc.

[0056] In some embodiments, the thickness of the duplex steel is ≤2.6 mm.

[0057] Dual-phase steel with a thickness ≤ 2.6mm allows for precise control of the cold rolling reduction rate and annealing speed to maximize lightweight and thinning benefits, ensuring both precision deep drawing and dent resistance for high-strength body panels. For example, the thickness of the dual-phase steel can be 0.6mm, 1.6mm, 2.6mm, etc.

[0058] The product prepared by the method of preparing duplex steel is the duplex steel described above. Since the method of preparing duplex steel adopts some or all of the technical solutions of the duplex steel embodiments, it has at least all the beneficial effects brought about by the technical solutions of the above embodiments, which will not be elaborated here.

[0059] The present application is further illustrated below with reference to specific embodiments. Experimental methods in the following embodiments that do not specify specific conditions are generally determined according to national / industry standards; if there is no corresponding national / industry standard, they are performed according to general international standards, conventional conditions, or conditions recommended by the manufacturer.

[0060] The chemical composition (mass percentage / %) of the examples and comparative examples is shown in Table 1.

[0061] Table 1

[0062] Based on the chemical composition of the examples and comparative examples, this embodiment also provides a method for preparing 980MPa grade duplex steel, including the following steps: A slab having the chemical composition described in Table 1 was obtained; The slab is sequentially heated and precision rolled to obtain hot-rolled steel strip; The hot-rolled steel strip is sequentially cooled, coiled, and heat-treated to obtain duplex steel. The process parameters for preparation are shown in Tables 2 and 3.

[0063] The process parameters for the examples and comparative examples are shown in Table 2.

[0064] Table 2

[0065] The heat treatment process parameters for the examples and comparative examples are shown in Table 3.

[0066] Table 3

[0067] The mechanical properties of the embodiments and comparative examples are shown in Table 4.

[0068] Table 4

[0069] The data tables above provide a clear comparison of the differences between various embodiments and comparative examples. The following conclusions can be drawn: As can be seen from the data in Table 4, the duplex steel provided in this application embodiment has a yield strength of 754MPa~788MPa, a tensile strength of 1002MPa~1020MPa, an elongation of 9.5%~13%, and a hole expansion rate of 35~38%.

[0070] As shown in Examples 1-5 and Comparative Examples 1-2, Examples 1-5, through the synergistic effect of optimized composition system and controlled rolling and cooling processes, precisely stabilized the yield strength within the target range of 700MPa~850MPa, while maintaining a comprehensive performance balance of tensile strength ≥1000MPa, elongation A80 ≥9.5%, and porosity ≥35%. By linking the hot rolling coiling temperature and the annealing rapid cooling temperature, the dual-phase microstructure was refined and the increase in yield strength ratio was suppressed, thus overcoming the problem of controlling springback during high-strength steel stamping. In Comparative Examples 1-2, although traditional molybdenum microalloying was used to achieve similar tensile strength and porosity, the excessively high coiling temperature led to microstructure coarsening, resulting in excessive yield strength and deteriorated yield strength ratio, ultimately failing to meet the precision forming requirements of automotive exterior body panels.

[0071] Appendix Figures 2-3 Detailed explanation: Figure 2 The image shows the tissue morphology taken using the electron microscope equipment provided in Embodiment 4 of this application. According to... Figure 2 It can be seen that the raised structure with white edges is martensite. During the phase transformation, Mn element is enriched at the grain boundaries, forming a white Mn-rich halo. The large black depressions are mainly ferrite.

[0072] Figure 3 This is a tissue morphology image taken using the light microscope apparatus provided in Embodiment 4 of this application. According to... Figure 3 It can be seen that the black part is mainly martensite, and the white part is mainly ferrite.

[0073] One or more technical solutions in the embodiments of the present invention have at least the following technical effects or advantages: The present invention provides a 980MPa grade dual-phase steel, which, through synergistically optimized composition design combined with precise hot rolling control and galvanizing process regulation, significantly reduces alloy costs while ensuring that the material has high yield strength and excellent formability, meeting the dual requirements of automotive lightweighting for high-strength steel mechanical properties and industrial mass production.

[0074] The above description is merely a specific embodiment of this application, enabling those skilled in the art to understand or implement this application. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of this application. Therefore, this application is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features claimed in this application.

Claims

1. A 980MPa grade duplex steel, characterized in that, The chemical composition of the duplex steel, by mass fraction, is as follows: C: 0.11%~0.13%, Si: 0.30%~0.40%, Mn: 2.40%~2.60%, Cr: 0.50%~0.70%, Alt: 0.02%~0.05%, Nb: 0.01%~0.03%, Ti: 0.02%~0.03%, P≤0.015%, S≤0.007%, with the remainder being Fe and unavoidable impurities; The microstructure of the dual-phase steel includes ferrite and martensite.

2. The duplex steel according to claim 1, characterized in that, The volume fraction of ferrite is 50% to 60%, and the volume fraction of martensite is 40% to 50%.

3. The duplex steel according to claim 1, characterized in that, The duplex steel meets at least one of the following properties: yield strength: 700MPa~850MPa, tensile strength ≥980MPa, elongation ≥8%.

4. A method for preparing duplex steel according to any one of claims 1 to 3, characterized in that, The method includes: A slab having the chemical composition described in any one of claims 1 to 3 is obtained; The slab is sequentially heated and precision rolled to obtain hot-rolled steel strip; The hot-rolled steel strip is sequentially cooled, coiled, and heat-treated to obtain duplex steel.

5. The method according to claim 4, characterized in that, The furnace exit temperature is 1240℃~1270℃, and the holding time is 160min~180min.

6. The method according to claim 4, characterized in that, The finishing rolling temperature is 860℃~900℃; and / or, The thickness of the hot-rolled steel strip is 2.5mm to 5.5mm.

7. The method according to claim 4, characterized in that, The cooling process employs ultra-fast medium-pressure cooling and front-end dense cooling, with a cooling rate of 25℃ / s to 35℃ / s.

8. The method according to claim 4, characterized in that, The winding temperature is 530℃~570℃.

9. The method according to claim 4, characterized in that, The heating temperature of the heat treatment is 795℃~805℃, the soaking temperature of the heat treatment is 795℃~805℃, and the slow cooling temperature of the heat treatment is 640℃~660℃; and / or, The rapid cooling temperature of the heat treatment is 300℃~320℃, and the over-aging temperature of the heat treatment is 290℃~310℃; and / or, The belt speed for the heat treatment is 70 m / min to 150 m / min, and the finishing elongation of the heat treatment is 0.2% to 0.4%.

10. The method according to claim 4, characterized in that, The thickness of the duplex steel is ≤2.6mm.

Images