A titanium microalloyed high yield strength 780mpa grade cold rolled dual phase steel with low coiling temperature and a method for producing the same

Abstract

The present application relates to the production technology field of advanced high-strength steel for automobiles, and particularly relates to a titanium micro-alloyed high yield strength 780MPa grade cold-rolled dual-phase steel adopting low-temperature coiling and a production method thereof. The chemical components in the steel are as follows in percentage by weight: C: 0.11% to 0.14%, Si: 0.35% to 0.55%, Mn: 1.80% to 2.20%, P: ≤0.015%, S: ≤0.008%, Ti: 0.020% to 0.040%, Als: 0.025% to 0.050%, N ≤0.004%, and the balance of Fe and inevitable impurities. The present application simplifies the micro-alloying system, reduces the cost and improves the purity, optimizes the hot-rolling temperature parameters, realizes the energy consumption and quality balance, cooperates with continuous annealing and small roller leveling, and improves the performance stability.

Description

Technical Field

[0001] This invention relates to the field of advanced high-strength steel production technology for automobiles, and in particular to a cold-rolled dual-phase steel with high yield strength of 780MPa using low-temperature coiling and its production method. Background Technology

[0002] HC500 / 780DP, a typical 780MPa grade cold-rolled dual-phase steel, is characterized by a microstructure of "soft ferrite + hard martensite." It needs to possess a yield strength of ≥500MPa, a tensile strength of ≥780MPa, and an elongation of ≥18% to balance the load-bearing capacity and cold formability of automotive structures. Currently, the production of this steel grade faces three major bottlenecks in industrial production, making it difficult to simultaneously achieve performance, cost, and production stability: Microalloying systems are complex, costly, and prone to defects. Existing technologies often employ "Ti+Nb" or "Ti+V" composite microalloying, which refines grains and strengthens through multi-element synergy. However, the addition of composite alloys increases the cost per ton of steel by 40-60 yuan, and Nb / V easily forms coarse carbides (≥5μm in size) with C in the steel, becoming a source of fatigue crack initiation and reducing the fatigue life of the finished product by 15-20%. Some single-Ti microalloying schemes, due to improper control of Ti content (≤0.015%), cannot fully fix N element (N≥0.006%), easily forming AlN inclusions and causing pitting defects on the surface of cold-rolled sheets. High hot rolling temperature parameters lead to a significant contradiction between energy consumption and quality. Conventional processes use slab exit temperatures of 1220-1280℃. While some existing technologies can ensure sufficient heating of the slab, they can easily result in coarse austenite grains (average grain size ≥30μm). This leads to uneven martensite distribution after subsequent cold rolling and continuous annealing, resulting in yield strength fluctuations of ±30MPa in the finished product. Meanwhile, coiling temperatures are generally controlled at 600-650℃. Excessively high coiling temperatures can cause coarse TiC (size ≥3μm) to precipitate in the ferrite matrix, weakening the precipitation strengthening effect. This requires an additional cold rolling reduction rate (≥65%) to compensate for the strength, resulting in a 10-15% increase in mill load and an 8-10kWh / ton increase in energy consumption. The lack of connection between continuous annealing and leveling processes results in poor performance stability. In existing technologies, leveling after continuous annealing often adopts a process of "high reduction rate combined with ordinary work rolls". Although it can eliminate plate shape defects, it is easy to cause martensitic phase breakage, which reduces the elongation by 2-3 percentage points. Some solutions omit leveling after continuous annealing, resulting in residual stress of ≥200MPa in the finished product and a cracking rate of 5-8% during cold forming, which cannot meet the forming requirements of complex automotive parts.

[0003] Existing technology 200680047438.9, "High-strength Duplex Steel with Low Yield Ratio, High Toughness and Excellent Weldability," has a high coiling temperature, resulting in coarse precipitates. This requires a large reduction rate in cold rolling compensation, increasing mill load and energy consumption. It is difficult to balance 780MPa yield strength and stable formability, and performance deviations are easily caused by process fluctuations. It is difficult to achieve a precise match between high yield strength and low residual stress, making it unsuitable for the requirements of complex automotive parts. Existing technology 202310338157.4, "A High Yield Strength 780MPa Grade Cold-Rolled Duplex Steel and Its Production Method," relies on multi-element composite microalloying (including Nb, Cr, Mo, etc.), which is costly and easily introduces coarse precipitates. Its flattening elongation is 0.3-0.8%, and it is prone to high residual stress. Furthermore, its high coiling temperature easily leads to coarse TiC, requiring a 50-70% cold rolling reduction rate for compensation. The existing technology 201710419254.0 "Manufacturing method of cold-rolled duplex steel with high yield strength of 780MPa" relies on the "C-Si-Mn-Cr-Nb" composite system. Nb and Cr increase the cost and are prone to the formation of coarse carbides. Its elongation is only 10-15% and the yield strength ratio is low.

[0004] In summary, existing technologies cannot achieve the synergy of "single Ti microalloying + low-energy hot rolling + stable performance", and there is an urgent need for a preparation method of HC500 / 780DP that is suitable for large-scale production, cost-controllable and meets performance standards. Summary of the Invention

[0005] This invention provides a method for producing a titanium microalloyed high yield strength 780MPa cold-rolled duplex steel with low-temperature coiling and its production. It simplifies the microalloying system, reduces costs and improves purity, optimizes hot rolling temperature parameters, and achieves a balance between energy consumption and quality. It also improves performance stability through synergistic continuous annealing and small roll leveling.

[0006] To achieve the above objectives, the present invention employs the following technical solution: A cold-rolled dual-phase steel with high yield strength of 780MPa, produced by low-temperature coiling and microalloyed titanium, has the following chemical composition by weight percentage: C: 0.11%–0.14%, Si: 0.35%–0.55%, Mn: 1.80%–2.20%, P: ≤0.015%, S: ≤0.008%, Ti: 0.020%–0.040%, Als: 0.025%–0.050%, N ≤0.004%, with the balance being Fe and unavoidable impurities, the total content of unavoidable impurities being ≤0.05%.

[0007] The yield strength is 500–550 MPa, the tensile strength is 780–830 MPa, and the elongation is 18%–22%.

[0008] The microstructure of the steel plate consists of fine-grained ferrite with a grain size of 8–12 μm and dispersed martensite, with a volume fraction of 25%–30% for the martensite.

[0009] A method for producing high-yield-strength (780 MPa) cold-rolled duplex steel with titanium microalloying using low-temperature coiling includes hot metal pretreatment, steelmaking and continuous casting, hot rolling, pickling rolling, continuous annealing and leveling; the specific method is as follows: 1) Hot metal pretreatment: A two-step pretreatment process of "desulfurization-dephosphorization" is adopted to meet the purity requirements of subsequent steelmaking: Desulfurization: CaO-CaF2 based desulfurizing agent (CaO / CaF2=8:1, addition amount 3-5kg / t iron) is added to molten iron at 1250-1300℃, and mechanical stirring is carried out (speed 180-200r / min) to control the S of molten iron after desulfurization to ≤0.005% and the desulfurization efficiency ≥90%.

[0010] Dephosphorization: A top-bottom combined blowing dephosphorization device is used, and FeO-CaO-SiO2 composite dephosphorizing agent is added (CaO / FeO=1.2~1.5, addition amount 8~10kg / t iron) to control the P of molten iron after dephosphorization to ≤0.010% and the dephosphorization efficiency ≥85%.

[0011] Temperature control: The temperature of molten iron is maintained at 1280-1320℃ after pretreatment to ensure that no additional heating is required during converter smelting, thus reducing energy consumption.

[0012] 2) Steelmaking and Continuous Casting: Converter smelting: A 120t top and bottom combined blowing converter is used, with a scrap steel ratio of ≤12wt%, an oxygen blowing time of 18-22min, an endpoint temperature of 1620-1650℃, and an endpoint C content controlled at 0.08-0.10% (to avoid over-oxidation leading to increased oxygen content).

[0013] External refining: An LF refining furnace is used, with argon gas introduced for stirring (flow rate 0.8–1.2 Nm³). 3 Ti iron (70% Ti content, 0.3-0.5 kg / t steel) is added in stages to ensure uniform dissolution of Ti; Si-Ca wire (0.2-0.3 kg / t steel) is added in the later stage of refining to control the total oxygen content ≤25 ppm, and the refining time is 30-35 min.

[0014] Continuous casting: A straight-arc continuous casting machine (crystallizer size 230mm×1250mm) is used. The casting sequence is preferably matched with steel grades of the same composition as HC500 / 780DP. The water flow rate in the crystallizer is 280-320m³. 3 / h, casting speed 1.2~1.5m / min; using "immersion nozzle + argon gas sealing" to protect the casting and avoid secondary oxidation; the surface of the billet is free of cracks (depth ≤1mm) and subcutaneous bubbles (depth ≤2mm), and the central porosity is ≤1.0 grade.

[0015] 3) Hot rolling includes: Furnace control: A walking beam furnace is used, with a weakly oxidizing atmosphere in the furnace chamber, an excess air coefficient of 1.05 to 1.15, a slab exit temperature of 1170 to 1230℃, and a heating time of 2.5 to 3.0 hours (ensuring uniform internal temperature with a temperature difference ≤25℃); by reducing the exit temperature, austenite grain growth is suppressed (average grain size ≤20μm).

[0016] Rolling parameters: 5 passes for roughing (exit thickness 30-35mm), 7 passes for finishing (exit thickness 3.0-5.0mm), final rolling temperature 850-880℃ (to ensure that austenite does not precipitate ferrite first), rolling speed 10-12m / s; after finishing rolling, a front-stage cooling mode is adopted, with a cooling rate of 20-25℃ / s, to quickly reduce the strip temperature to the phase transformation zone.

[0017] Low-temperature coiling: coiling temperature 530~570℃, coiling tension 18~22kN; low-temperature coiling and low slab exit temperature work together to suppress coarse precipitation of TiC in ferrite (TiC size ≤2μm) and refine austenite grains, while ensuring uniform nucleation of martensite during coiling. The microstructure of hot-rolled coil is "fine-grained ferrite + a small amount of martensite + retained austenite", with a retained austenite volume fraction of 5~8%.

[0018] This invention combines a low furnace exit temperature of 1200±30℃ with a low-temperature coiling temperature of 550±20℃, which reduces energy consumption and lays a fine-grained foundation for subsequent continuous annealing microstructure control, unlike the conventional "high furnace exit + high coiling" process.

[0019] 4) Pickling and rolling process: Uncoiling and welding: Hot-rolled coils are uncoiled by an uncoiler (tension 12-15kN) and adjacent coils are connected by laser welding (welding speed 1.8-2.0m / min, weld strength ≥ 90% of the base material) to avoid cold rolling cracking at the weld.

[0020] Pickling: Hydrochloric acid pickling is used, with a pickling solution concentration of 18wt%~22wt%, a temperature of 80~85℃, and a pickling time of 90~110s to remove iron oxide scale from the surface of hot-rolled coils (residual amount ≤5mg / m). 2 After pickling, high-pressure water rinsing (pressure 12-15MPa) + hot air drying (temperature 120-150℃) is used to avoid acid residue.

[0021] Cold rolling: A five-stand cold rolling mill is used, with a cold rolling reduction rate of 55% to 65% (adjusted according to the finished product thickness: 60% reduction rate for 1.5mm finished product, and 55% reduction rate for 2.0mm finished product); rolling speed of 300 to 500 m / min, rolling tension of 100 to 150 kN (lower for thinner specifications, higher for thicker specifications); cold rolled sheet thickness tolerance ≤ ±0.02mm, sheet shape deviation ≤ 5I, and work hardening hardness HV280 to 320.

[0022] 5) Continuous rolling and leveling process: Continuous annealing: A vertical continuous annealing furnace is used, with four temperature control sections; Heating section: temperature 820~840℃, holding time 60~80s, to ensure that the residual austenite is fully transformed into martensite; Slow cooling section: final temperature 670~710℃, cooling rate 5~7℃ / s, to avoid rapid martensite transformation leading to increased internal stress; Rapid cooling section: final temperature 290~340℃, cooling rate 30~40℃ / s, to fix the martensite structure and inhibit TiC precipitation; Over-aging section: 260~290℃, holding time 30~40s (to eliminate internal stress and stabilize the structure).

[0023] Small roll leveling: A four-roll leveling machine is used, with fine-grained alloy rolls for the work rolls. The roughness Ra is 0.6-0.8μm, the diameter is 500mm, and the leveling reduction rate is 0.3-0.5% (0.5% for 1.0-1.3mm, 0.4% for 1.4-1.7mm, and 0.3% for 1.8-2.0mm). The rolling tension is 80-120kN, and the rolling speed is 400-600m / min. After leveling, the residual stress of the cold-rolled sheet is ≤150MPa, the surface roughness Ra is ≤0.8μm, and there is no waviness (wavy height ≤1mm / m) or warping (warping degree ≤1.5mm / m).

[0024] This invention combines continuous annealing with a four-stage temperature control mode and small roller leveling (low reduction rate + fine grain alloy roller), which eliminates work hardening and avoids martensite breakage, ensuring a balance between strength and plasticity. This is different from conventional "high reduction leveling" or "no leveling" processes, resulting in a finished product residual stress ≤150MPa.

[0025] This invention targets HC500 / 780DP steel grade, and the entire process is designed around the core logic of "single Ti microalloying + low hot rolling temperature + low temperature coiling + continuous annealing - small roll leveling synergy". The rationale for the chemical composition design is as follows: The carbon content is 0.11% to 0.14%, which provides a carbon source for the martensitic phase transformation and ensures the basic strength without excessive carbon content that would reduce formability.

[0026] A Si content of 0.35%–0.55% promotes the enrichment of C and Mn from ferrite into austenite, thereby increasing the activity of C in ferrite.

[0027] The Mn content is 1.80% to 2.20%, which expands the austenite region, delays the martensitic transformation, and refines the martensitic structure.

[0028] P≤0.015%, reducing grain boundary segregation, avoiding cold brittleness, and controlling low-temperature impact energy loss.

[0029] S≤0.008%, reducing MnS inclusion formation and avoiding fatigue crack initiation sources.

[0030] Ti content is 0.020% to 0.040%, which fixes N (forming TiN with a size ≤1μm), refines grains (prevents austenite growth), and strengthens by precipitation (forming TiC with a size of 1-2μm).

[0031] Al content 0.025%~0.050%, assists in deoxygenation, fixes residual O (total oxygen ≤25ppm), and avoids AlN inclusions.

[0032] N≤0.004, control the amount of TiN generated to avoid excessive TiN leading to a decrease in toughness.

[0033] In terms of composition design, this invention can achieve the triple function of "solid N + fine grains + precipitation" using only Ti (0.020%~0.040%), replacing composite microalloying, reducing costs while avoiding multi-element interaction defects.

[0034] Compared with the prior art, the beneficial effects of the present invention are: 1. Simplify the microalloying system, reduce costs and improve purity: By microalloying Ti (without Nb / V addition), the Ti content is controlled to achieve the triple effect of "fixing N + refining grains + precipitation strengthening". N in steel ≤ 0.004%, carbide size ≤ 2μm, surface defect rate of cold-rolled sheet ≤ 1.5%, and first-pass yield ≥ 96%.

[0035] Reduced microalloying costs: Adding only Ti (0.020-0.040%) reduces the cost of alloy steel by 30-40 yuan per ton, which is 57-55.6% lower than the cost of "Ti+Nb" composite microalloying (70-90 yuan / ton).

[0036] 2. Optimize hot rolling temperature parameters to achieve a balance between energy consumption and quality: The slab exit temperature is controlled at 1200±30℃ (about 50℃ lower than the conventional temperature), and the coiling temperature is controlled at 550±20℃ (50-100℃ lower than the conventional temperature). While ensuring sufficient austenite phase transformation, the austenite grain size of the hot-rolled coil is ≤20μm, and the energy consumption per ton of hot rolling is reduced by 12-15kWh.

[0037] Low furnace exit temperature and low temperature coiling reduce the energy consumption per ton of hot rolling steel by 12-15 kWh. Based on an industrial electricity price of 0.6 yuan / kWh, this translates to a cost saving of 7.2-9 yuan per ton of steel.

[0038] 3. Synergistic continuous annealing and small roller leveling enhance performance stability: A "continuous annealing with precise temperature control + small roller leveling" integrated process is designed to ensure the finished product's microstructure is "fine-grained ferrite (8-12μm particle size) + dispersed martensite (25-30% volume fraction)". Mechanical properties meet the following requirements: yield strength 500-550MPa, tensile strength 780-830MPa, elongation 18-22%, and cold forming cracking rate ≤2%. There are no coarse carbides (TiC ≤ 2μm) or AlN inclusions, resulting in a 20-25% improvement in fatigue life (based on 10). 7 In the alternating load test, the stress amplitude of the finished product of this invention reached 350MPa, while the stress amplitude of the existing composite microalloying scheme is about 280-290MPa. The test standard refers to GB / T30768-2022.

[0039] 4. Adaptable to existing production lines, reducing the threshold for transformation: No new core equipment (such as vacuum degassing devices or special rolling mills) is required. Only the temperature control system of the heating furnace and the parameters of the leveling mill roll system need to be adjusted. The transformation cost is ≤300,000 yuan, which is 99.4% lower than that of building a new production line (investment ≥50 million yuan), and is suitable for the industrialized mass production needs of small and medium-sized steel plants.

[0040] 5. Compared with the prior art, the present invention has significantly improved strength-plasticity balance and stability. Specific parameter comparisons are shown in Table 1 below. Table 1 Comparison of performance parameters of the present invention and prior art: This invention optimizes the composition and process, while ensuring that the tensile strength meets the 780MPa requirement, and appropriately reduces the tensile strength to significantly improve the elongation and yield strength ratio, thus achieving a better balance between strength and plasticity.

[0041] 6. Application scenarios: The finished product can be directly used in automotive door anti-collision beams (1.5mm thickness) and chassis crossbeams (2.0mm thickness). It does not crack after cold forming, and its collision resistance (energy absorption value ≥50kJ / m) meets the requirements of GB / T30768-2022 "High-strength cold-rolled steel sheets and strips for automobiles". Attached Figure Description

[0042] Figure 1 This is a 500x metallographic microstructure image corresponding to Example 1.

[0043] Figure 2 This is a 500x metallographic microstructure image corresponding to Example 2. Detailed Implementation

[0044] The present invention will be further described in detail below with reference to the embodiments, but the scope of protection of the present invention is not limited thereto. Experimental methods for which specific conditions are not specified in the embodiments are generally determined according to national / industry standards; if there is no corresponding national / industry standard, then they are performed according to general international standards, conventional conditions, or conditions recommended by the manufacturer.

[0045] The chemical composition and mass percentage of the examples are shown in Table 2; the hot rolling process of the examples is shown in Table 3; the cold rolling process with pickling of the examples is shown in Table 4; the continuous annealing and leveling process of the examples is shown in Table 5; and the mechanical properties and microstructure of the examples are shown in Table 6.

[0046] Example 1 was used to process a front door anti-collision beam for automobiles (cross-section size 40mm×20mm). After a collision resistance test (impact speed 5m / s), the energy absorption value reached 52kJ / m, a 15.6% improvement over the existing solution (45kJ / m), meeting automotive safety collision regulations. The surface roughness Ra was 0.7μm, and no cracks were observed during cold forming (bending diameter = 1.5 times thickness, 180°). Example 2 was used to process a chassis crossbeam for automobiles (length 1200mm). After a static load test (load 30kN), the maximum deformation was ≤2mm, with no plastic deformation. After a 1000h salt spray test (neutral salt spray), the corrosion rate was ≤0.02mm / a, suitable for outdoor automotive use. The surface roughness Ra was 0.8μm, and the fatigue life (10... 7 The alternating load reaches 350 MPa (stress amplitude).

[0047] Table 2 Chemical composition of the examples (%): Table 3 Smelting and hot rolling process parameters for the examples: Table 4. Pickling and cold rolling process parameters for the examples: Table 5 Examples of continuous annealing and leveling processes: Table 6 Mechanical properties and microstructure of the embodiments: Samples were prepared according to GB / T13298-2015. They were water-cooled cut using a metallographic cutting machine, clamped, and ground with 180#, 320#, 600#, and 800# sandpaper. The samples were then coarsely polished with a 3.5μm polishing agent, followed by a 0.5μm fine polishing. Finally, they were etched with 4% nitric acid alcohol for 4 seconds to prepare the metallographic structure. Grain size classification was performed according to GB / T6394-2017; non-metallic inclusion classification in steel was performed according to GB / T10561-2005 (Method A); and second phase area content measurement was performed according to GB / T18876.1-2002.

[0048] Figure 1 The 500x metallographic microstructure image corresponding to Example 1 clearly shows the typical "fine-grained ferrite + dispersed martensite" dual-phase structure of the cold-rolled dual-phase steel of the present invention, which is highly consistent with the product's design goal of "strength-plastic balance + stable formability", providing key microstructure guarantee for its adaptation to the application requirements of automotive front door anti-collision beams. In this structure, the matrix phase is uniformly distributed equiaxed fine-grained ferrite with regular grain outlines and no grain agglomeration or abnormal growth. Image analysis shows that the average grain diameter is about 9.0 μm (within the design range of 8-12 μm). This fine-grained structure is due to the inhibitory effect of the low furnace temperature of 1200℃ on austenite grain growth and the pinning effect of the fine TiN particles formed by Ti elements. Through grain boundary strengthening, the yield strength and cold forming ability of the finished product are significantly improved. The martensite phase is uniformly dispersed in the ferrite grain boundaries and within the grains in the form of islands and strips, without continuous network or agglomeration defects, and the volume fraction reaches 28% (in line with the target range of 25-30%). This state is achieved by low-temperature coiling at 550℃ to promote uniform nucleation of martensite and continuous annealing heating section (820℃, 70s) to ensure the full transformation of residual austenite. The second phase strengthening ensures that the tensile strength is stable and meets the standard. Meanwhile, the control of non-metallic inclusions in the steel is excellent, with a point inclusion (DT) rating of 0.5 and a sulfide inclusion (DS) rating of 0.5. There are no coarse TiC or AlN inclusions ≥2μm, which effectively eliminates fatigue crack initiation sources. Combined with the synergistic effect of fine grains and dispersed phases, the finished product is crack-free when cold-formed (bending mandrel diameter = 1.5 times the thickness, 180°), and the impact energy absorption value reaches 52kJ / m, which fully meets the structural load-bearing and safety protection requirements of automotive front door anti-collision beams.

[0049] Figure 2The 500x metallographic microstructure of the HC500 / 780DP grade cold-rolled duplex steel prepared by the core process of the present invention in Example 2 is consistent with the design logic of the present invention of "achieving a balance between strength and plasticity by synergistically controlling the structure through the whole process", providing key structural support for the finished product to meet the performance standards. The microstructure exhibits a typical dual-phase characteristic of "fine-grained ferrite + dispersed martensite": the matrix phase is equiaxed ferrite with clear grain outlines and uniform size, without abnormal grain growth or deformation, and a grain size grade of 10 (corresponding to an average grain diameter of approximately 8.0 μm). This fine-grained structure originates from the inhibitory effect of low slab tapping temperature on austenite grain growth, and the grain refinement effect of Ti element through the formation of fine TiN particles, which can significantly improve the yield strength of the finished product by relying on grain boundary strengthening; the martensite phase is uniformly dispersed in island and strip shapes at the ferrite grain boundaries and within the grains, without agglomeration or continuous network distribution defects. Image analysis shows that its volume fraction is 28%, which falls entirely within the target range of "25-30%" of this invention. This state is achieved by low-temperature coiling promoting uniform martensite nucleation and continuous annealing heating ensuring full transformation of residual austenite, which can ensure a stable tensile strength of 780 MPa through second-phase strengthening. Meanwhile, the non-metallic inclusions in the steel are extremely low, with point inclusions (DT) rated at 1.0 and sulfide inclusions (DS) rated at 0.5. There are no coarse TiC or AlN inclusions ≥2μm. This is due to the sufficient fixation of N by Ti and the control of harmful elements by molten iron pretreatment, which eliminates fatigue crack initiation sources.

[0050] This invention presents an industrial method for preparing 780MPa grade cold-rolled duplex steel (grade HC500 / 780DP) solely through titanium microalloying, combined with low slab exit temperature, low-temperature coiling, and continuous annealing followed by small-roll leveling. This steel grade is primarily used in automotive body structural components (such as door anti-collision beams and A-pillar reinforcement plates) and safety components (such as chassis crossbeams), meeting the requirements for lightweighting and collision resistance in automobiles. It is particularly suitable for low-cost retrofitting and upgrading of existing cold-rolled duplex steel production lines.

Claims

1. A cold-rolled duplex steel with high yield strength of 780 MPa, microalloyed titanium, produced by low-temperature coiling, characterized in that... The chemical composition of the steel, by weight percentage, is as follows: C: 0.11%~0.14%, Si: 0.35%~0.55%, Mn: 1.80%~2.20%, P: ≤0.015%, S: ≤0.008%, Ti: 0.020%~0.040%, Als: 0.025%~0.050%, N≤0.004%, with the balance being Fe and unavoidable impurities.

2. The titanium microalloyed high yield strength 780MPa cold-rolled duplex steel with low-temperature coiling as described in claim 1, characterized in that, The yield strength is 500–550 MPa, the tensile strength is 780–830 MPa, and the elongation is 18%–22%.

3. The titanium microalloyed high yield strength 780MPa cold-rolled duplex steel with low-temperature coiling as described in claim 1, characterized in that, The microstructure of cold-rolled steel sheet is: fine-grained ferrite with a particle size of 8-12 μm + dispersed martensite, with a volume fraction of 25%-30% for martensite.

4. A method for producing titanium microalloyed high yield strength 780MPa cold-rolled duplex steel using low-temperature coiling as described in any one of claims 1-3, comprising hot metal pretreatment, steelmaking and continuous casting, hot rolling, pickling rolling, continuous annealing and leveling; characterized in that, The hot rolling process includes: slab exit temperature of 1170-1230℃, heating time of 2.5-3.0h; final rolling temperature of 850-880℃; pre-cooling mode after finishing rolling, cooling rate of 20-25℃ / s; and coiling temperature of 530-570℃.

5. The method for producing titanium microalloyed high yield strength 780MPa grade cold-rolled duplex steel using low-temperature coiling according to claim 4, characterized in that, The continuous retreat and leveling include: Heating section: temperature 820~840℃; slow cooling section final temperature: 670~710℃, cooling rate 5~7℃ / s; rapid cooling section final temperature: 290~340℃, cooling rate 30~40℃ / s; over-aging section: 260~290℃; leveling reduction rate 0.3%~0.5%.

6. The method for producing titanium microalloyed high yield strength 780MPa grade cold-rolled duplex steel using low-temperature coiling according to claim 4, characterized in that, In the hot rolling process, the furnace atmosphere of the heating furnace is weakly oxidizing, and the excess air coefficient is 1.05 to 1.

15.

7. The method for producing titanium microalloyed high yield strength 780MPa grade cold-rolled duplex steel using low-temperature coiling according to claim 4, characterized in that, The molten iron pretreatment includes: controlling the sulfur content of the molten iron after desulfurization to be ≤0.005%, the phosphorus content of the molten iron after dephosphorization to be ≤0.010%, and maintaining the temperature of the molten iron after pretreatment at 1280~1320℃.

8. The method for producing titanium microalloyed high yield strength 780MPa grade cold-rolled duplex steel using low-temperature coiling according to claim 4, characterized in that, The steelmaking and continuous casting processes include: a converter smelting endpoint temperature of 1620–1650℃, with the endpoint C content controlled at 0.08–0.10%; and a continuous casting crystallizer water flow rate of 280–320 m³ / h. 3 / h, pulling speed 1.2~1.5m / min.

9. The method for producing titanium microalloyed high yield strength 780MPa grade cold-rolled duplex steel using low-temperature coiling according to claim 4, characterized in that, The pickling process includes: Pickling: Hydrochloric acid is used for pickling, with a pickling solution concentration of 18wt%~22wt%, a temperature of 80~85℃, and a pickling time of 90~110s. After pickling, high-pressure water rinsing and hot air drying are used. Cold rolling: cold rolling reduction rate 55%~65%; rolling speed 300~500m / min.

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