780mpa grade enhanced formability cold rolled dual phase steel and method of making the same

Abstract

The present application relates to the technical field of high-strength steel production, and particularly relates to a 780MPa grade enhanced formability cold-rolled dual-phase steel and a preparation method thereof, wherein the chemical composition of the steel is C: 0.09% to 0.11%, Si: 0.25% to 0.45%, Mn: 1.9% to 2.1%, P: less than or equal to 0.012%, S: less than or equal to 0.006%, Als: 0.03% to 0.05%, N: less than or equal to 0.005%, Cr: 0.25% to 0.35%, Nb: 0.015% to 0.025%, Cu: 0.08% to 0.12%, Ni: 0.05% to 0.09%, and Nb / Cr=1:10 to 1:12; Ceq=0.38% to 0.42%, and the rest is Fe and impurities; the core target of the present application is to make the work hardening index n value greater than or equal to 0.24 through "component synergistic optimization + process precise regulation", so as to fundamentally solve the industry pain points of the existing 780MPa grade cold-rolled dual-phase steel, such as insufficient hole expansion performance and poor formability, while reducing the alloy cost and improving the manufacturability.

Description

Technical Field

[0001] This invention relates to the field of high-strength steel production technology, and in particular to a 780MPa grade reinforced formable cold-rolled duplex steel and its preparation method. Background Technology

[0002] With the development of the automotive industry, the requirements for lightweight and safety in automobiles are increasing. Advanced high-strength steel, as a key material for achieving lightweight automobiles, has been widely used in the automotive manufacturing field. As the first generation of advanced high-strength steel, dual-phase steel remains the first choice for advanced high-strength steel in automobiles due to its high strength, low yield strength ratio, and good formability. Among them, 780MPa grade cold-rolled dual-phase steel is mainly used to manufacture automotive structural parts and reinforcements.

[0003] However, traditional 780MPa grade cold-rolled duplex steel has shortcomings in hole-expanding performance. When used to manufacture automotive parts, especially during processes involving flanging and hole expansion, the steel sheet is prone to cracking. This is because the microstructure of traditional duplex steel consists of softer ferrite and harder martensite. Cracks easily initiate and propagate at the ferrite-martensite phase interface, and the hard, brittle martensite also tends to initiate cracks.

[0004] Chinese patent application No. 202110173209.8 discloses "A cold-rolled annealed duplex steel with enhanced formability and its preparation method." The chemical composition of the duplex steel, by mass fraction, is as follows: C: 0.15%–0.20%, Si: 0.60%–1.20%, Mn: 1.8%–2.3%, Al: 0–1.0%, Cr: 0.15%–0.25%, Ni: 0–0.01%, Cu: 0–0.01%, Mo: 0–0.005%, Nb: 0–0.005%, V: 0–0.00%. 0.005%, Ti: 0~0.005%, B: 0~0.0005%, P: 0~0.01%, S: 0~0.001%, with the remainder being Fe and unavoidable impurities; its continuous annealing aging section temperature is only 280~400℃, which can achieve C distribution, but fails to promote the precipitation of fine carbides from martensite, resulting in poor ferrite-martensite interface bonding, which cannot suppress crack initiation during pore expansion. Moreover, its C content is 0.15%~0.20%, and the high carbon content easily leads to a decrease in plasticity and weldability, and increases the yield strength ratio.

[0005] Chinese patent application No. 202010928563.2 discloses "a 780MPa grade high-plasticity cold-rolled DH steel and its preparation method". The steel contains C: 0.10%~0.18%, Mn: 1.5%~2.5%, Si: 0.4%~0.8%, Al: 0.02%~0.7%, Cr: 0.02%~0.50%, P≤0.01%, S≤0.01%, Nb≤0.1%, Ti≤0.1%, and Si+Al: 0.5%~1.5%, with the balance being iron and unavoidable impurities. The rolling temperature is 1050–1150℃, the final rolling temperature is ≥900℃, and the coiling temperature is 550–700℃; the cold rolling reduction rate is 40%–80%; the continuous annealing preheating temperature is 200–600℃, the annealing temperature is 760–880℃, the annealing time is 10–600s, the slow cooling exit temperature is 660–760℃, the rapid cooling rate is >20℃ / s, the over-aging temperature is 300–420℃, and the over-aging time is 30–3600s; the finishing elongation is 0.3%–0.7%. The requirement for a rapid cooling rate greater than 20℃ / s easily leads to uneven martensite distribution and poor ferrite-martensite interface bonding, resulting in a work hardening index (n) of only 0.20–0.21, insufficient formability, and a tendency to crack during the hole expansion process. Summary of the Invention

[0006] This invention provides a 780MPa grade enhanced formability cold-rolled duplex steel and its preparation method. The core objective is to achieve a work hardening index n value ≥ 0.24 through "composition synergistic optimization + precise process control", fundamentally solving the industry pain points of insufficient hole expansion performance and poor formability of existing 780MPa grade cold-rolled duplex steel, while reducing alloy costs and improving manufacturability.

[0007] To achieve the above objectives, the present invention employs the following technical solution:

[0008] A 780MPa grade reinforced formable cold-rolled duplex steel has the following chemical composition by mass percentage: C: 0.09%–0.11%, Si: 0.25%–0.45%, Mn: 1.9%–2.1%, P≤0.012%, S≤0.006%, Als: 0.03%–0.05%, N≤0.005%, Cr: 0.25%–0.35%, Nb: 0.015%–0.025%, Cu: 0.08%–0.12%, Ni: 0.05%–0.09%, where Nb / Cr = 0.083–0.1; carbon equivalent Ceq = 0.432%–0.497%, and Ceq = C + Mn / 6 + (Cr + Cu + Ni) / 15, with the remainder being Fe and unavoidable impurities; the tensile strength R of the steel plate is... m The yield strength is 800-850 MPa, and the yield strength R is... el The strength is 450–510 MPa, and the yield strength ratio R is...el / R m =0.56~0.60; Elongation A 80 The value is 22%–25%; the porosity λ is 50%–65%; the V-notch impact absorption energy at -40℃ is ≥30J; the hardness is 220–250HV; and the work hardening index n value is ≥0.24.

[0009] The microstructure of the finished steel plate consists of ferrite, martensite and Mao islands, with the volume ratio of martensite and Mao islands being ≥46%; the size of Mao islands is ≤2μm, and the grain size grade is ≥12.5.

[0010] A method for preparing 780MPa grade reinforced formable cold-rolled duplex steel includes the following production process:

[0011] 1) Hot metal pretreatment: Hot metal is pretreated for desulfurization and dephosphorization to reduce the sulfur content to below 0.005% and the phosphorus content to below 0.01%;

[0012] 2) Steelmaking and continuous casting:

[0013] (1) Converter steelmaking: The pretreated molten iron and scrap steel are added to the converter and oxygen is blown to carry out decarburization reaction. The carbon content at the end of the converter steelmaking is controlled at 0.09% to 0.11%, and the temperature of the molten steel at the end is controlled at 1650 to 1680℃. During the blowing process, slag-forming agent is added to form alkaline slag to remove phosphorus and sulfur impurities from the molten steel.

[0014] (2) Refining: The combined LF and RH refining process is adopted; during the LF refining process, refining slag is added for deoxidation, desulfurization and removal of inclusions. After LF refining, the sulfur content of the molten steel is ≤0.002%; after LF refining, the Al content of the molten steel is controlled at 0.02% to 0.06%, the total oxygen content T[O] of the molten steel is ≤0.003%, and after RH vacuum degassing, the [H] in the molten steel is ≤0.00025% and [N] is ≤0.006%.

[0015] (3) Continuous casting: The billet pulling speed is controlled at 1.0 to 1.5 m / min, and the continuous casting cooling rate is controlled at 0.8 to 1.5 ℃ / s;

[0016] 3) Hot rolling:

[0017] (1) Heating: Heat the continuously cast slab to 1150-1250℃ for 1.5-2.5 hours;

[0018] (2) Rough rolling: The initial rolling temperature of rough rolling shall not be lower than 1100℃, and the total reduction rate of rough rolling shall be ≥94% after multiple rolling passes;

[0019] (3) Finishing rolling: The initial rolling temperature of finishing rolling is 950-1050℃, and the final rolling temperature of finishing rolling is controlled at 850-900℃; the total reduction rate of finishing rolling is ≥66.7%, and the total reduction rate of hot rolling is 98.0%-98.5%;

[0020] (4) Laminar flow cooling: The cooling rate is controlled at 20-40℃ / s;

[0021] (5) Winding: The winding temperature is controlled at 600-650℃;

[0022] 4) Pickling and rolling:

[0023] (1) Pickling: The pickling temperature is controlled at 70-80℃;

[0024] (2) Cold rolling: The cold rolling reduction rate is controlled at 60% to 70%;

[0025] 5) Continuous annealing:

[0026] (1) Heating section: A segmented induction heating system is adopted. In the initial stage, the temperature is heated to 400-450℃ at a low speed of 5-8℃ / s and held for 15-20s. Then, the temperature is heated to 780-820℃ at a high speed of 15-22℃ / s. During the heating process, the heating rate fluctuation is controlled to not exceed ±2℃ / s, and the temperature deviation of the strip along the thickness direction is ≤5℃.

[0027] (2) Soaking zone: Hold at 780-820℃ for 60-120s. During the holding process, control the furnace atmosphere to suppress oxidation of the strip surface; control the austenite volume fraction in the steel to be 30%-40%.

[0028] (3) Cooling section: A composite cooling system of "water mist cooling + jet cooling" is adopted. The strip steel is first cooled from the homogenization temperature to 390-430℃ at a rapid cooling rate of 30-50℃ / s. During the cooling process, the water mist pressure and jet flow rate are dynamically adjusted to ensure the uniformity of strip steel cooling. The strip steel is held at 390-430℃ for 5-10s.

[0029] (4) Over-aging stage: The strip steel is heated to 450-500℃ at a heating rate of 8-12℃ / s and held for 30-60s;

[0030] (5) Smoothing: The smoothing elongation rate is controlled at 0.8% to 1.2%.

[0031] During LF refining, CaO-Al2O3-based refining slag with an alkalinity R of 3 to 5 is added; and 1.5 to 2.0 kg / t of silicon-aluminum-barium-calcium deoxidizer is added.

[0032] The finishing mill adopts a 7-stand continuous rolling method.

[0033] Pickling is performed using hydrochloric acid pickling process, with a hydrochloric acid mass concentration of 15% to 20%.

[0034] During continuous annealing, the furnace atmosphere in the soaking zone consists of nitrogen and hydrogen, with the volume fraction of hydrogen being ≥5%.

[0035] During continuous annealing, the water mist pressure in the cooling section is 0.3–0.5 MPa, and the jet flow rate is 500–800 m³ / h. 3 / h.

[0036] Before leveling, the strip is pre-adjusted for tension, with a front tension of 150–200 MPa and a back tension of 180–230 MPa. The working roll pressure during leveling is 800–1200 kN, and the rolling speed is 300–500 m / min. The strip shape deviation after leveling is ≤5I, and the surface roughness of the strip is 0.8–1.2 μm.

[0037] Compared with the prior art, the beneficial effects of the present invention are:

[0038] 1) Increase the hole expansion rate:

[0039] This invention optimizes the composition design, rationally controls the content of each alloying element, and adopts specific preparation processes (such as rapid cooling and over-aging treatment during continuous annealing) to make the microstructure of duplex steel more uniform and the interface between ferrite and martensite better. This effectively inhibits the initiation and propagation of cracks at the phase interface, thereby significantly improving the porosity of duplex steel (the porosity can reach 50% to 65%, which is a significant improvement compared to traditional 780MPa grade cold-rolled duplex steel).

[0040] 2) Enhanced formability:

[0041] The core advantage lies in the optimized n-value (≥0.24), which significantly improves the uniform work hardening capacity of the steel during stamping, flanging, and hole expansion. Compared to traditional 780MPa duplex steel (n-value = 0.20~0.22), it reduces the risk of cracking caused by localized stress concentration by more than 80%, directly meeting the forming requirements of complex automotive structural parts without additional adjustments to the processing technology. The finished steel sheet has a low yield strength ratio and high elongation, enabling it to better adapt to complex deformations during the stamping and forming processes of automotive parts, reducing defects such as cracking and wrinkling, improving formability, and meeting the high-quality forming requirements of the automotive industry for parts.

[0042] 3) Reduce alloy costs:

[0043] Compared with duplex steels that contain a large amount of expensive alloying elements (such as high content of Mo, Nb, etc.), this invention reduces the amount of expensive alloying elements used by rationally optimizing the alloy system, thereby effectively reducing alloy costs and improving the market competitiveness of the product while ensuring performance.

[0044] 4) Improve manufacturability:

[0045] In the steelmaking and continuous casting processes, the quality of molten steel and cast billets is ensured and defects in the production process are reduced through hot metal pretreatment, appropriate converter steelmaking and refining processes, and optimization of continuous casting process parameters. Precise control of process parameters in subsequent processes such as hot rolling, pickling, continuous annealing, and leveling makes the entire production process more stable, improves production efficiency and product quality stability, and facilitates large-scale industrial production. Attached Figure Description

[0046] Figure 1 This is a metallographic photograph of the 780MPa grade reinforced formability cold-rolled duplex steel prepared in Example 1 of the present invention. Detailed Implementation

[0047] The present invention discloses a 780MPa grade reinforced formable cold-rolled dual-phase steel. The chemical composition of the steel, by mass percentage, is as follows: C: 0.09%–0.11%, Si: 0.25%–0.45%, Mn: 1.9%–2.1%, P≤0.012%, S≤0.006%, Als: 0.03%–0.05%, N≤0.005%, Cr: 0.25%–0.35%, Nb: 0.015%–0.025%, Cu: 0.08%–0.12%, Ni: 0.05%–0.09%, wherein Nb / Cr = 0.083–0.1; carbon equivalent Ceq = 0.432%–0.497%, and Ceq = C + Mn / 6 + (Cr + Cu + Ni) / 15, with the remainder being Fe and unavoidable impurities. The rationale for selecting the roles and content ranges of each chemical component in the steel is as follows:

[0048] Carbon (C) is a key element affecting the strength and plasticity of duplex steel. If the C content is too low, the amount of austenite formed during critical zone annealing will be insufficient and its stability will be inadequate, making it difficult to guarantee both strength and plasticity. If the C content is too high, the plasticity and weldability of the duplex steel will decrease. This invention controls the C content at 0.09%–0.11%, which can maintain good plasticity and weldability while ensuring strength.

[0049] Si: Silicon is a ferrite solid solution strengthening element that can significantly improve the strength of steel plates. It can also promote the enrichment of carbon atoms from ferrite to austenite, purify ferrite, inhibit the precipitation of carbides at the galvanizing temperature, and improve the stability of austenite. However, excessive Si content can affect the plating susceptibility of the substrate. Therefore, this invention controls the Si content to be 0.25% to 0.45%.

[0050] Mn: Manganese can improve the stability of austenite, shift the C-curve to the right, and reduce the critical cooling rate of martensite. Manganese is also a good deoxidizer and desulfurizer. However, excessive Mn content can affect the solderability of the substrate and the quality of surface zinc plating. Therefore, this invention controls the Mn content to 1.9%–2.1%.

[0051] P and S: Phosphorus and sulfur are harmful impurity elements in steel. Excessive content will reduce the toughness and weldability of steel. This invention strictly controls P≤0.012% and S≤0.006%.

[0052] Als: Acid-soluble aluminum can refine grains and improve the toughness of steel. In this invention, the Als content in the steel is controlled to be 0.03% to 0.05%.

[0053] N: Excessive nitrogen content can lead to age hardening of steel and reduce its plasticity. This invention controls the N content to be ≤0.005%.

[0054] Cr: Chromium can refine the grain structure, inhibit grain coarsening during hot working, promote carbon diffusion into austenite, improve austenite stability, and reduce the critical cooling rate during annealing. However, excessive Cr content can impair the ductility of steel. This invention controls the Cr content to be 0.25%–0.35%.

[0055] Ni: 0.05%~0.09%, which counteracts the hot brittleness tendency of Cu, refines the martensite structure, and improves the bonding between ferrite and martensite.

[0056] This invention controls the Nb / Cr ratio in steel to be between 0.083 and 0.1. Through the synergistic effect of the two, Nb (C,N) and Cr complement each other in refining the hot-working microstructure, maximizing the effect of suppressing grain coarsening. If the ratio exceeds the set range, either excessive Nb will lead to deterioration of hot-working performance, or insufficient Cr will prevent grain refinement, thus failing to achieve the target grain size ≥12.5.

[0057] Niobium (Nb) strongly inhibits dynamic recrystallization by combining with C and N to form Nb(C,N), which suppresses grain coarsening during hot working and refines ferrite grains. However, adding excessive Nb will deteriorate the hot working properties and toughness of steel plates. Therefore, this invention controls the Nb content to be 0.015% to 0.025%.

[0058] Cu: It can strengthen ferrite through microalloying solid solution, improve the corrosion resistance and strength matching of steel, and avoid hot working brittleness caused by high content. In this invention, the Cu content is controlled to be 0.08% to 0.12%.

[0059] With a carbon equivalent of 0.432% to 0.497%, the balance between weldability and strength and plasticity of steel can be precisely controlled by adjusting the carbon equivalent. Too high a carbon equivalent can easily lead to cold cracking during welding, while too low a carbon equivalent cannot guarantee the strength.

[0060] The preparation process of the 780MPa grade reinforced formable cold-rolled duplex steel described in this invention is as follows:

[0061] I. Hot metal pretreatment:

[0062] Desulfurization and dephosphorization pretreatment is performed on molten iron. The KR stirring desulfurization method is preferred, where a desulfurizing agent (such as a CaO-CaF2-based desulfurizer) is added to the molten iron ladle, and the molten iron is vigorously stirred with a stirrer to ensure sufficient contact and reaction between the desulfurizing agent and the molten iron, reducing the sulfur content in the molten iron to below 0.005%. For dephosphorization, an oxidizing agent (such as Fe2O3) and a flux (such as CaO) are added to the molten iron, and the dephosphorization reaction is carried out at a suitable temperature (1300–1350℃) to reduce the phosphorus content in the molten iron to below 0.01%. This molten iron pretreatment provides high-quality molten iron with low sulfur and low phosphorus content for subsequent steelmaking, reducing the impact of harmful impurities in the steel on its performance.

[0063] II. Steelmaking and Continuous Casting:

[0064] 1. Converter steelmaking: Pretreated molten iron is added to the converter along with an appropriate amount of scrap steel. Oxygen is blown in for decarburization. During the blowing process, a slagging agent is added to form alkaline slag, which removes phosphorus and sulfur impurities from the molten steel. The intensity and time of oxygen blowing are controlled, and the carbon content at the end of the converter steelmaking process is controlled at 0.06% to 0.08%, and the end steel temperature is controlled at 1650 to 1680℃.

[0065] The final carbon content in the converter is controlled at 0.06%–0.08%, which is 0.01%–0.03% lower than the finished product carbon content (0.09%–0.11%). During the LF refining process, a slight increase in carbon (approximately 0.01%–0.02%) is achieved by adding low-carbon alloy materials with a carbon content ≤0.05% and controlling carbon precipitation in refractory materials. At the same time, there is a slight loss in RH vacuum decarburization (decarburization ≤0.005%). Finally, through fine-tuning of the composition during the refining process, the carbon content of the finished product is precisely controlled to 0.09%–0.11%, which conforms to the actual operation logic of "low carbon extraction in the converter + fine-tuning of carbon in refining" in industrial production. This avoids the problems of composition fluctuation and low carbon control precision caused by directly extracting carbon to the finished product in the converter.

[0066] 2. Refining: The molten steel after converter smelting undergoes refining treatment using a combined LF (Ladle Refining Furnace) and RH (Vacuum Circulating Degassing Unit) refining process. During LF refining, refining slag is added for deoxidation, desulfurization, and inclusion removal. CaO-Al2O3-based refining slag with a basicity R=3-5 is preferred. Additionally, 1.5-2.0 kg / t of silicon-aluminum-barium-calcium deoxidizer is added. This achieves a three-stage deoxidation process: pre-deoxidation → diffusion deoxidation → precipitation deoxidation, preventing secondary inclusion formation and effectively removing inclusions. After LF refining, the sulfur content of the molten steel is ≤0.002%; the Al content is controlled at 0.02%-0.06%; the total oxygen content (T[O]) is ≤0.003%; and after RH vacuum degassing, the [H] content is ≤0.00025% and [N] is ≤0.006%. To achieve the dual goals of "deep degassing + prevention of secondary nitrogen absorption", the content of inclusions in steel is further reduced and the purity of molten steel is improved.

[0067] 3. Continuous Casting: After refining, the molten steel meets the conditions for continuous casting, preferably using an arc-shaped continuous casting machine. Control the continuous casting process parameters (such as billet speed and cooling intensity). The billet speed is adjusted according to the steel grade and the cross-sectional dimensions of the billet, generally controlled between 1.0 and 1.5 m / min. Weak cooling is used, with the continuous casting cooling rate controlled between 0.8 and 1.5℃ / s to ensure the solidification quality of the billet and reduce internal defects. The preferred cross-sectional dimensions of the billet are 200mm × 1500mm. After cutting, the billet is sent to subsequent processes.

[0068] III. Hot Rolled:

[0069] 1. Heating: The continuously cast slab is heated to 1150–1250℃ for 1.5–2.5 hours. By controlling the heating temperature and time, the internal structure of the slab is made homogenized, and the carbides are fully dissolved, creating favorable conditions for subsequent rolling.

[0070] 2. Hot Rolling: Roughing is preferably performed using a reversible roughing mill to roll the heated slab into an intermediate billet. The initial roughing temperature is not lower than 1100℃, and the total roughing reduction is ≥94% (e.g., if the original thickness of a continuously cast slab is 200mm, the intermediate billet thickness after multiple rolling passes is ≤12mm). This large reduction breaks down the as-cast microstructure, laying the foundation for subsequent fine grain formation. The intermediate billet then enters the finishing mill for finishing rolling. Precise control of the initial and final rolling temperatures ensures a uniform steel plate microstructure. The initial finishing temperature is 950–1050℃, and the final rolling temperature is controlled between 850–900℃. Finishing rolling preferably uses a 7-stand continuous rolling method. Precise control of the reduction and rolling speed of each stand ensures good plate shape and dimensional accuracy. The total finishing reduction is ≥66.7% (e.g., if a 12mm thick intermediate billet is finished to a thickness of 3.0–4.0mm). The overall reduction rate of hot rolling is 98.0% to 98.5%, which is in line with the actual production of high-strength steel hot rolling with large reduction and fine grain refinement in large-scale industrial production, and avoids the problems of coarse grains and uneven structure caused by small reduction rates.

[0071] 3. Laminar Flow Cooling: The finished steel plate is cooled by a laminar flow cooling system, with the cooling rate controlled between 20 and 40°C / s. The cooling rate and cooling mode are adjusted according to the steel grade and product performance requirements to allow the steel plate to undergo phase transformation within a suitable temperature range, forming the desired microstructure.

[0072] 4. Coiling: The cooled steel sheet is coiled into a hot-rolled coil, with the coiling temperature controlled between 600 and 650℃. A suitable coiling temperature helps control the microstructure and properties of the steel sheet, improving the quality of the hot-rolled coil.

[0073] IV. Pickling and rolling:

[0074] 1. Pickling: The hot-rolled coil is fed into the pickling unit, and hydrochloric acid pickling is preferably used to remove the iron oxide scale from the surface of the hot-rolled coil. The pickling temperature is controlled at 70-80℃, and the hydrochloric acid mass concentration is 15%-20%. By adjusting the pickling time and acid concentration, the iron oxide scale is ensured to be completely removed, while avoiding over-pickling that may damage the steel plate surface.

[0075] 2. Cold Rolling: After pickling, the hot-rolled sheet enters the cold rolling mill for cold rolling, with the cold rolling reduction rate controlled at 60%–70%. Multi-stand cold rolling reduces the sheet thickness while simultaneously improving its strength and surface quality. During cold rolling, parameters such as rolling speed and tension are controlled to ensure the stability of the process and the quality of the sheet shape.

[0076] V. Continuous annealing:

[0077] A vertical continuous annealing furnace is preferred, and the microstructure and performance optimization of cold-rolled strip steel are achieved through continuous annealing. The entire annealing process is precisely controlled by temperature, time, and cooling rate to achieve the refined preparation of the two-phase microstructure, which is divided into the following four core stages:

[0078] 1. Heating Section: A segmented induction heating system is adopted. In the initial stage, the strip is heated at a low speed of 5-8℃ / s to 400-450℃ and held for 15-20s to eliminate the processing stress generated during cold rolling and prevent deformation of the strip due to stress concentration. Then, the strip is heated at a high speed of 15-22℃ / s to 780-820℃. During this stage, the heating rate is strictly controlled to ensure that the fluctuation does not exceed ±2℃ / s and to ensure that the temperature deviation of the strip along the thickness direction is ≤5℃, so as to prevent abnormal grain growth due to local overheating.

[0079] 2. Soaking Zone: The strip is held at 780–820℃ for 60–120 seconds. During this holding period, the furnace atmosphere is controlled (e.g., nitrogen + hydrogen, with hydrogen content ≥ 5%) to suppress surface oxidation. This stage requires sufficient recrystallization and partial austenitization of ferrite, with the austenite volume fraction controlled at 30%–40%. Austenite primarily nucleates at ferrite grain boundaries, forming fine, uniform island-like distributions, laying the microstructure foundation for subsequent phase transformation.

[0080] 3. Cooling Section: A combined cooling system of "water mist cooling + jet cooling" is adopted. First, the strip steel is cooled from the homogenization temperature to 390–430℃ (near the martensitic transformation completion temperature Ms) at a rapid cooling rate of 30–50℃ / s. During the cooling process, the water mist pressure (0.3–0.5 MPa) and jet flow rate (500–800 m³ / s) are dynamically adjusted. 3 / h), to ensure uniform cooling of the strip and avoid uneven martensitic structure caused by fluctuations in cooling rate; hold at 390~430℃ for 5~10s to allow the martensitic phase transformation to be fully completed, while reducing the internal stress of phase transformation.

[0081] 4. Over-aging stage: The strip steel is heated to 450-500℃ at a heating rate of 8-12℃ / s and held for 30-60s. This stage can promote the precipitation of a small amount of carbon in martensite, forming fine carbides, reducing the hardness of martensite, improving the interfacial bonding state between ferrite and martensite, eliminating the internal stress generated during cooling, optimizing the fine grain + uniform dual-phase structure, further improving the work hardening index n value, enhancing the plasticity, toughness and formability of the strip steel, and avoiding cracking during subsequent processing.

[0082] 5. Leveling: A four-roll leveler is preferred for leveling cold-rolled strip. Before leveling, the strip tension is pre-adjusted (pre-tension 150–200 MPa, post-tension 180–230 MPa) to ensure stable entry of the strip into the leveler. The leveling elongation is precisely controlled between 0.8% and 1.2%. By adjusting the work roll pressure (800–1200 kN) and rolling speed (300–500 m / min), the strip shape is optimized (shape deviation ≤ 5 μm). Simultaneously, the surface roughness of the strip is controlled between 0.8 and 1.2 μm to meet the surface quality requirements of subsequent processing.

[0083] The mechanical properties of the 780MPa grade reinforced formable cold-rolled dual-phase steel described in this invention are as follows:

[0084] 1. Tensile strength (R) m ): 800~850MPa, ensuring that the steel grade meets the strength requirements of 780MPa high-strength steel, and meets the load-bearing capacity requirements of automotive structural components.

[0085] 2. Yield strength (R) el ): 450~510MPa, yield strength ratio (Rel / Rm) controlled at 0.56~0.60; a low yield strength ratio can improve the formability of steel and avoid cracking caused by excessive yield strength during stamping.

[0086] 3. Elongation (A) 80 ): 22%~25%, the high elongation rate can ensure that the steel plate has good plastic deformation ability in complex forming process.

[0087] 4. Hole expansion rate (λ): 50%~65%, which is significantly improved compared to the traditional 780MPa grade cold-rolled duplex steel (the hole expansion rate is usually 35%~45%), and can effectively solve the cracking problem in flanging and hole expansion processing.

[0088] 5. Impact absorption energy (-40℃, V-notch): ≥30J, ensuring that the steel plate still has good toughness in low-temperature environments and meets the safety requirements of automotive low-temperature use scenarios.

[0089] 6. Hardness (HV): 220~250HV, where tensile strength of 800~820MPa corresponds to hardness HV220~230, and tensile strength of 820~850MPa corresponds to hardness HV230~250. Hardness and strength have a good linear positive correlation, which ensures the strength of steel while avoiding the increase in processing difficulty due to excessive hardness.

[0090] The above mechanical properties were tested in accordance with standards such as GB / T228.1-2021 "Metallic materials - Tensile testing - Part 1: Test method at room temperature", GB / T5168-2021 "Metallic materials - Hole expansion test method", and GB / T229-2020 "Metallic materials - Charpy pendulum impact test method".

[0091] Unless otherwise specified, all contents in this invention are mass contents.

[0092] To more intuitively illustrate the present invention, the embodiments of the present invention will be further described in conjunction with the examples. The following examples are merely preferred embodiments of the present invention, but the scope of protection of the present invention is not limited thereto. Any technical solutions that can be obviously obtained by those skilled in the art within the scope of the technology disclosed in the present invention, including simple variations or equivalent substitutions, are all within the scope of protection of the present invention.

[0093] Example 1:

[0094] In this embodiment, the preparation process of 780MPa grade reinforced formable cold-rolled dual-phase steel is as follows:

[0095] 1. Composition Design: The chemical composition of the steel by mass percentage is as follows: C: 0.105%, Si: 0.45%, Mn: 1.95%, P: 0.012%, S: 0.006%, Als: 0.05%, N: 0.005%, Cr: 0.26%, Nb: 0.023%, Cu: 0.10%, Ni: 0.07%, Nb / Cr = 0.088, Ceq = 0.458%, with the remainder being Fe and unavoidable impurities.

[0096] 2. Hot metal pretreatment: KR stirring desulfurization method is adopted, with a desulfurizing agent addition of 8 kg / t hot metal, stirring speed of 150 r / min, stirring time of 12 min, and the sulfur content of hot metal is reduced to 0.003%; during the dephosphorization process, Fe2O3 (3 kg / t hot metal) and CaO (5 kg / t hot metal) are added, and the hot metal is kept at 1350℃ for 20 min, and the phosphorus content of hot metal is reduced to 0.009%.

[0097] 3. Steelmaking and Continuous Casting: During converter steelmaking, the ratio of molten iron to scrap steel is 8:2, and the oxygen blowing intensity is 3.5 m. 3 / (t・min), final carbon content 0.08%, molten steel temperature 1680℃; during LF refining, CaO-Al2O3 based refining slag (5kg / t molten steel) was added, refining time 40min, molten steel sulfur content decreased to 0.002%, carbon slightly increased to C=0.11%; RH vacuum degassing vacuum degree ≤67Pa, treatment time 25min, molten steel hydrogen content decreased to 0.00018%; continuous casting adopted arc continuous casting machine, billet speed 1.5m / min, cooling water volume 1200L / min, billet cross-sectional size 200mm×1500mm, billet internal defect rate ≤0.5%.

[0098] 4. Hot Rolling: The slab is heated to 1250℃ in a walking beam furnace for 2.5 hours. The furnace atmosphere is controlled to be weakly oxidizing (oxygen volume fraction 3%–5%). Roughing is performed in 10 passes with an initial rolling temperature of 1150℃, rolling a 200mm thick cast slab to a 10mm thick intermediate slab, with a total roughing reduction of 95%. Finishing is performed using a 7-stand continuous rolling mill with an initial rolling temperature of 1050℃ and a final rolling temperature of 900℃, rolling a 10mm thick intermediate slab to a 3.0mm thick hot-rolled plate, with a total finishing reduction of 70%; the total hot rolling reduction is 98.5%. Laminar flow cooling uses a front-end rapid cooling mode with a cooling rate of 40℃ / s and a coiling temperature of 650℃.

[0099] 5. Pickling and rolling: A continuous hydrochloric acid pickling line is used. The pickling tank temperature is 80℃, the hydrochloric acid concentration is 20%, and the pickling time is 60s. After pickling, the iron oxide scale removal rate on the strip surface is ≥99.5%. Cold rolling is carried out using a 5-stand cold rolling mill. The total cold rolling reduction rate is 70%, the rolling speed is 800m / min, and the finished thickness of the cold-rolled sheet is 1.2mm.

[0100] 6. Continuous annealing and leveling: A segmented induction heating system is adopted. In the initial stage, the strip is heated to 450℃ at a low speed of 8℃ / s and held for 18s. Then, it is heated to 820℃ at a high speed of 20℃ / s (the heating rate fluctuation is controlled to not exceed ±2℃ / s during the heating process, and the temperature deviation of the strip along the thickness direction is ≤5℃). The heat soaking time is 120s, and the austenite volume fraction is 40%. A "water mist cooling + jet cooling" method is used, with a cooling rate of 50℃ / s to cool to 420℃. After aging, the strip is heated to 500℃ at a heating rate of 12℃ / s and held for 60s. The leveling elongation is 1.2%, the strip shape deviation after leveling is 3I, and the surface roughness is 1.2μm.

[0101] 7. Performance Testing: The tensile strength R of the duplex stainless steel finished product was tested. m =850MPa (corresponding to HV250), yield strength R el =510MPa, yield strength ratio 0.60, elongation A 80=22%, porosity λ=65%. Work hardening index n=0.26, -40℃ V-notch impact absorption energy=35J; the microstructure of the finished product is ferrite + martensite + Mao islands, of which the volume ratio of martensite to Mao islands is 46%, the grain size grade is 12.5, and the inclusion grade is DT1.0.

[0102] 8. Application effect: The cold-rolled dual-phase steel prepared in this embodiment is used in the manufacture of automobile chassis crossbeams. The crossbeams need to undergo multiple flanging (flanging height 15mm) and hole enlargement (hole diameter ø20mm) during the processing. There is no cracking after processing, which meets the requirements of automobile chassis for load bearing and impact resistance.

[0103] Figure 1 Table 1 shows the metallographic microstructure (500x magnification) of the cold-rolled dual-phase steel prepared in this embodiment. The grain size and inclusion statistics are presented in Table 1. Evaluation was conducted according to GB / T6394-2017 "Method for Determination of Average Grain Size of Metals" and GB / T10561-2023 "Determination of Non-metallic Inclusion Content in Steel".

[0104] The measurement of the second phase area content was conducted according to GB / T18876.1-2002 "Standard Test Methods for Determination of Metallographic Structure, Inclusion Content and Grade in Steel and Other Metals by Automated Image Analysis - Part 1: Image Analysis and Stereoscopic Determination of Inclusion or Second Phase Structure Content in Steel and Other Metals". Sample preparation was performed according to GB / T13298-2015 "Metallic Microstructure Examination Methods". The samples were water-cooled and clamped using a metallographic cutting machine, and successively ground with 180#, 320#, 600#, and 800# sandpaper. They were then coarsely polished with 3.5μm polishing agent, followed by fine polishing with 0.5μm polishing agent, and etched with 4% nitric acid alcohol for 4 seconds.

[0105] Table 1. Statistical data on grain size and inclusions in metallographic structures;

[0106]

[0107] Example 1 verifies the method for preparing a 780MPa grade reinforced formable cold-rolled duplex steel according to the present invention, as follows:

[0108] I. Metallographic data confirms the effect of composition optimization: precise control of elements and their content lays a high-quality tissue foundation through synergistic effects.

[0109] One of the core technologies of this invention is the optimization of the alloy composition system (C: 0.09%–0.11%, Si: 0.25%–0.45%, Mn: 1.9%–2.1%, P≤0.012%, S≤0.006%, Als: 0.03%–0.05%, N≤0.005%, Cr: 0.25%–0.35%, Nb: 0.015%–0.025%, Cu: 0.08%–0.12%, Ni: 0.05%–0.09%, where Nb / Cr = 0.083–0.1; carbon equivalent Ceq = 0.432%–0.497%). The metallographic data of Example 1 directly verifies the effectiveness of this composition design.

[0110] 1. Cold-rolled dual-phase steel exhibits a fine-grained structure (grain size reaches grade 12.5), thanks to the synergistic refining effect of Nb and Cr. According to GB / T6394-2017 "Method for Determination of Average Grain Size of Metals", the higher the grain size grade, the finer the grains, proving that the precise control of Nb (0.023%) and Cr (0.26%) in the composition is effective; Nb inhibits the coarsening of hot-working grains by forming Nb(C,N), while Cr refines the hot-working structure, and the two work together to achieve ultra-fine ferrite grains.

[0111] 2. The microstructure exhibits a high proportion of martensite and austenite islands (46% in this embodiment), achieving controlled austenite stability in the C / Mn ratio. This corresponds to the process design of this invention, where the austenite volume fraction in the soaking section is 30%–40% (after cooling, austenite transforms into martensite + MA), demonstrating the precise ratio of C (0.105%) to Mn (1.95%) in the composition.

[0112] C can promote austenite enrichment and ensure strength; Mn can improve austenite stability and reduce the critical cooling rate of martensite, ultimately forming a dual-phase structure of "ferrite matrix + uniform island MA", avoiding the problem of stress concentration cracking caused by uneven martensite distribution in traditional dual-phase steel, and achieving enhanced formability (the yield strength ratio of the dual-phase steel in Example 1 is 0.60 and the elongation is 22%, which meets the complex stamping requirements of automobiles).

[0113] Second, the metallographic data confirms the precise control of the process, and the fine preparation of the microstructure is achieved through the synergy of multiple process stages.

[0114] Another core technology of this invention is the use of a full-process control of "continuous annealing + hot rolling + acid rolling". The metallographic data of Example 1 can verify the effectiveness of the key process control.

[0115] The cold-rolled duplex steel prepared in Example 1 exhibits a uniform "F+M+MA" microstructure, which is directly related to the "four-stage" control employed during continuous annealing. This involves a continuous annealing process of "heating to 820℃ + soaking for 120s + cooling at a rate of 50℃ / s + over-aging at 500℃ for 60s," resulting in a fine, uniform island-like distribution of MA in the microstructure (without bulk martensite). Specifically, in the heating and soaking stages, austenite nucleation at ferrite grain boundaries is precisely controlled to prevent martensite coarsening caused by localized overheating. In the cooling stage, a composite cooling method of "water mist + air jet" (50℃ / s) is used to ensure uniform martensite transformation. In the over-aging stage, carbon precipitation in the martensite is promoted to form fine carbides, reducing martensite hardness and improving the ferrite-martensite interface bonding (traditional duplex steel has poor interface bonding and is prone to cracking). Ultimately, the goal is to improve the uniformity of the structure, achieve a high porosity (15mm flange, 20mm porosity without cracking) and enhance the formability of the product.

[0116] Low inclusion (DT1.0 grade) smelting is achieved through hot metal pretreatment and refining processes. In this embodiment, the inclusion grade in the smelted steel reaches DT1.0 (according to GB / T10561-2023 "Determination of Non-metallic Inclusion Content in Steel" standard, which is an extremely low inclusion level), proving the effectiveness of the "KR desulfurization + LF + RH refining" process of this invention. Hot metal pretreatment desulfurizes to 0.003% and dephosphorizes to 0.009%, reducing harmful impurities; LF + RH refining removes hydrogen (1.8ppm), nitrogen, and inclusions, resulting in high steel purity. Low inclusion smelting not only avoids inclusions becoming crack sources but also reduces the risk of nozzle blockage during casting (solving the pain point of poor manufacturability of traditional high-Al steel), enabling stable mass production and improving manufacturability.

[0117] Third, the effectiveness of the preparation method described in this invention was demonstrated through closed-loop verification of "metallography-performance-application", which solved the problems of "low hole expansion rate, high cost and poor manufacturability" of traditional 780MPa grade cold-rolled duplex steel.

[0118] The microstructure determines the performance of the finished product: The cold-rolled duplex steel prepared in this embodiment has a microstructure of "fine grain (grade 12.5) + low inclusions (grade DT1.0) + 46% martensite and uniform MA (Mao island size ≤ 2μm)". This microstructure is the core reason for the n value reaching 0.26, and ultimately achieves the synergistic optimization of "tensile strength 850MPa + porosity 65% ​​+ n value 0.26", breaking through the technical bottleneck of "difficulty in balancing strength and formability" in traditional duplex steel.

[0119] Performance determines application: its practicality is verified through industrial application. The cold-rolled duplex steel prepared in this embodiment is used in automotive chassis crossbeams (requiring multiple flanging and hole enlargement), and no cracking occurs during processing, proving that the product can meet the actual needs of automotive parts manufacturing.

[0120] Example 2:

[0121] In this embodiment, the preparation process of 780MPa grade reinforced formable cold-rolled dual-phase steel is as follows:

[0122] Composition design: The chemical composition of the steel by mass percentage is C: 0.10%, Si: 0.25%, Mn: 1.90%, P: 0.010%, S: 0.004%, Als: 0.03%, N: 0.004%, Cr: 0.29%, Nb: 0.025%, Cu: 0.10%, Ni: 0.07%, Nb / Cr=0.086, Ceq=0.447%, with the remainder being Fe and unavoidable impurities.

[0123] 2. Hot metal pretreatment: KR stirring desulfurization method is adopted, with a desulfurizing agent addition of 6 kg / t hot metal, stirring speed of 140 r / min, stirring time of 10 min, and the sulfur content of hot metal is reduced to 0.0025%; Fe2O3 (2.5 kg / t hot metal) and CaO (4 kg / t hot metal) are added during the dephosphorization process, and the hot metal is held at 1330℃ for 18 min, and the phosphorus content of hot metal is reduced to 0.008%.

[0124] 3. Steelmaking and Continuous Casting: During converter steelmaking, the ratio of molten iron to scrap steel is 8.5:1.5, and the oxygen blowing intensity is 3.0 m. 3 / (t・min), final carbon content 0.06%, molten steel temperature 1650℃; during LF refining, CaO-Al2O3 based refining slag (4kg / t molten steel) was added, refining time 35min, molten steel sulfur content reduced to 0.0015%, carbon slightly increased to C=0.09%; RH vacuum degassing vacuum degree ≤67Pa, treatment time 20min, molten steel hydrogen content reduced to 0.00020%; continuous casting adopted arc continuous casting machine, billet speed 1.2m / min, cooling water volume 1000L / min, billet cross-sectional size 200mm×1500mm, billet internal defect rate ≤0.6%.

[0125] 4. Hot rolling: The slab is heated to 1150℃ in a walking beam furnace for 2.0 hours. The furnace atmosphere is controlled to be weakly oxidizing (oxygen volume fraction 3%~5%). The roughing rolling adopts 8 passes, with the roughing rolling start temperature at 1100℃. The 200mm thick cast slab is rolled to a 12mm thick intermediate slab, and the total reduction rate of the roughing rolling is 94%. The finishing rolling adopts 7 stands continuous rolling, with the finishing rolling start temperature at 950℃ and the finishing rolling finish temperature at 850℃. The 12mm thick intermediate slab is rolled to a 3.0mm thick hot-rolled plate, with a finishing rolling total reduction rate of 75% and an overall hot rolling total reduction rate of 98.5%. The laminar flow cooling adopts a front-end rapid cooling mode with a cooling rate of 25℃ / s and a coiling temperature of 620℃.

[0126] 5. Pickling and rolling: A continuous hydrochloric acid pickling line is used, with a pickling tank temperature of 75℃, a hydrochloric acid concentration of 18%, and a pickling time of 50s. After pickling, the iron oxide scale removal rate on the strip surface is ≥99.5%. Cold rolling is carried out using a 5-stand cold rolling mill, with a total cold rolling reduction rate of 65%, a rolling speed of 750m / min, and a finished thickness of 1.05mm for the cold-rolled sheet.

[0127] 6. Continuous Annealing and Leveling: A segmented induction heating system is used. In the initial stage, the strip is heated at a low speed of 6℃ / s to 420℃ and held for 15s. Subsequently, it is heated at a high speed of 15℃ / s to 800℃ (the heating rate fluctuation is controlled to not exceed ±2℃ / s during the heating process, and the temperature deviation of the strip along the thickness direction is ≤5℃). The soaking time is 90s, and the austenite volume fraction is 30%. A "water mist cooling + jet cooling" method is used, with a cooling rate of 30℃ / s, cooling to 390℃. For over-aging, the strip is heated to 450℃ at a heating rate of 8℃ / s and held for 30s. The leveling elongation is 0.8%, the strip shape deviation after leveling is 4I, and the surface roughness is 0.8μm.

[0128] 7. Performance Testing: The tensile strength R of the duplex stainless steel finished product was tested. m =800MPa (corresponding to HV220), yield strength R el =450MPa, yield strength ratio 0.56, elongation A 80 =25%, porosity λ=52%, work hardening index n=0.24, -40℃ V-notch impact absorption energy=32J; the finished product microstructure is ferrite + martensite + Mao islands, the volume ratio of martensite and Mao islands is 46%, the grain size grade is 13.0, and the inclusion grade is DT0.5.

[0129] 8. Application effect: The cold-rolled duplex steel prepared in this embodiment was applied to the stamping of automotive door reinforcement plates. The stamping process was smooth, with no cracking or wrinkling. The formed door reinforcement plates were of good quality and met the usage requirements.

[0130] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in the present invention, based on the technical solution and inventive concept of the present invention, should be covered within the scope of protection of the present invention.

Claims

1. A 780MPa grade reinforced formable cold-rolled dual-phase steel, characterized in that, The chemical composition of the steel, by mass percentage, is as follows: C: 0.09%–0.11%, Si: 0.25%–0.45%, Mn: 1.9%–2.1%, P≤0.012%, S≤0.006%, Als: 0.03%–0.05%, N≤0.005%, Cr: 0.25%–0.35%, Nb: 0.015%–0.025%, Cu: 0.08%–0.12%, Ni: 0.05%. ~0.09%, of which Nb / Cr=0.083~0.1; carbon equivalent Ceq=0.432%~0.497%, and Ceq=C+Mn / 6+(Cr+Cu+Ni) / 15, the remainder being Fe and unavoidable impurities; the microstructure of the finished steel plate consists of ferrite, martensite and Mao islands, of which the volume ratio of martensite and Mao islands is ≥46%; Mao island size ≤2μm, grain size grade ≥12.5; the tensile strength R of the steel plate m The yield strength is 800-850 MPa, and the yield strength R is... el The strength is 450–510 MPa, and the yield strength ratio R is... el / R m =0.56~0.60; Elongation A 80 The working hardness is 22% to 25%; the porosity λ is 50% to 65%; the V-notch impact absorption energy at -40℃ is ≥30J; the hardness is 220 to 250HV; and the work hardening index n value is ≥0.

24.

2. A method for preparing 780MPa grade reinforced formable cold-rolled duplex steel as described in claim 1, characterized in that, The production process includes the following: 1) Hot metal pretreatment: Hot metal is pretreated for desulfurization and dephosphorization to reduce the sulfur content to below 0.005% and the phosphorus content to below 0.01%; 2) Steelmaking and continuous casting: (1) Converter steelmaking: The pretreated molten iron and scrap steel are added to the converter and oxygen is blown to carry out decarburization reaction. The carbon content at the end of the converter steelmaking is controlled at 0.06% to 0.08%, and the temperature of the molten steel at the end is controlled at 1650 to 1680℃. During the blowing process, slag-forming agent is added to form alkaline slag to remove phosphorus and sulfur impurities from the molten steel. (2) Refining: The combined LF and RH refining process is adopted; during the LF refining process, refining slag is added for deoxidation, desulfurization and removal of inclusions. After LF refining, the sulfur content of the molten steel is ≤0.002%; after LF refining, the Al content of the molten steel is controlled at 0.02% to 0.06%, the total oxygen content T[O] of the molten steel is ≤0.003%, and after RH vacuum degassing, the [H] in the molten steel is ≤0.00025% and [N] is ≤0.006%. (3) Continuous casting: The billet pulling speed is controlled at 1.0 to 1.5 m / min, and the continuous casting cooling rate is controlled at 0.8 to 1.5 ℃ / s; 3) Hot rolling: (1) Heating: Heat the continuously cast slab to 1150-1250℃ for 1.5-2.5 hours; (2) Rough rolling: The initial rolling temperature of rough rolling shall not be lower than 1100℃, and the total reduction rate of rough rolling shall be ≥94% after multiple rolling passes; (3) Finishing rolling: The initial rolling temperature of finishing rolling is 950-1050℃, and the final rolling temperature of finishing rolling is controlled at 850-900℃; the total reduction rate of finishing rolling is ≥66.7%, and the total reduction rate of hot rolling is 98.0%-98.5%; (4) Laminar flow cooling: The cooling rate is controlled at 20-40℃ / s; (5) Winding: The winding temperature is controlled at 600-650℃; 4) Pickling and rolling: (1) Pickling: The pickling temperature is controlled at 70-80℃; (2) Cold rolling: The cold rolling reduction rate is controlled at 60% to 70%; 5) Continuous annealing: (1) Heating section: A segmented induction heating system is adopted. In the initial stage, the temperature is heated to 400-450℃ at a low speed of 5-8℃ / s and held for 15-20s. Then, the temperature is heated to 780-820℃ at a high speed of 15-22℃ / s. During the heating process, the heating rate fluctuation is controlled to not exceed ±2℃ / s, and the temperature deviation of the strip along the thickness direction is ≤5℃. (2) Soaking zone: Hold at 780-820℃ for 60-120s. During the holding process, control the furnace atmosphere to suppress oxidation of the strip surface; control the austenite volume fraction in the steel to be 30%-40%. (3) Cooling section: A composite cooling system of "water mist cooling + jet cooling" is adopted. The strip steel is first cooled from the homogenization temperature to 390-430℃ at a rapid cooling rate of 30-50℃ / s. During the cooling process, the water mist pressure and jet flow rate are dynamically adjusted to ensure the uniformity of strip steel cooling. The strip steel is held at 390-430℃ for 5-10s. (4) Over-aging stage: The strip steel is heated to 450-500℃ at a heating rate of 8-12℃ / s and held for 30-60s; (5) Smoothing: The smoothing elongation rate is controlled at 0.8% to 1.2%.

3. The method for preparing 780MPa grade reinforced formable cold-rolled duplex steel according to claim 2, characterized in that, During LF refining, CaO-Al2O3-based refining slag with an alkalinity R of 3 to 5 is added; and 1.5 to 2.0 kg / t of silicon-aluminum-barium-calcium deoxidizer is added.

4. The method for preparing 780MPa grade reinforced formable cold-rolled dual-phase steel according to claim 2, characterized in that, The finishing mill adopts a 7-stand continuous rolling method.

5. The method for preparing 780MPa grade reinforced formable cold-rolled duplex steel according to claim 2, characterized in that, Pickling is performed using hydrochloric acid pickling process, with a hydrochloric acid mass concentration of 15% to 20%.

6. The method for preparing 780MPa grade reinforced formable cold-rolled dual-phase steel according to claim 2, characterized in that, During continuous annealing, the furnace atmosphere in the soaking zone consists of nitrogen and hydrogen, with the volume fraction of hydrogen being ≥5%.

7. The method for preparing 780MPa grade reinforced formable cold-rolled dual-phase steel according to claim 2, characterized in that, During continuous annealing, the water mist pressure in the cooling section is 0.3–0.5 MPa, and the jet flow rate is 500–800 m³ / h.

8. The method for preparing 780MPa grade reinforced formable cold-rolled dual-phase steel according to claim 2, characterized in that, Before leveling, the strip is pre-adjusted for tension, with a front tension of 150–200 MPa and a back tension of 180–230 MPa. The working roll pressure during leveling is 800–1200 kN, and the rolling speed is 300–500 m / min. The strip shape deviation after leveling is ≤5I, and the surface roughness of the strip is 0.8–1.2 μm.

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