Vanadium-titanium microalloyed cold-rolled TRIP780 steel and production method thereof

Abstract

The present application relates to advanced high-strength steel material and preparation technical field, especially to a kind of cold-rolled TRIP780 steel containing vanadium titanium micro alloy and production method thereof.Suitable for new energy automobile key structural parts, energy storage equipment shell and so on manufacturing.Chemical composition in steel is as follows by weight percentage: C:0.18%~0.22%, Si:0.8%~1.2%, Mn:1.6%~2.0%, V:0.02%~0.08%, P:≤0.015%, S≤0.005%, Ti:0.01%~0.05%, Als:0.02%~0.05%, N≤0.006%, and V / Ti=2~4, Ti / N≥3.42, Mn / C=8~10;The balance is Fe and inevitable impurities.The present application improves residual austenite stability by developing vanadium titanium synergistic microalloying system;Optimize low-temperature rolling process, reduce energy consumption.

Description

Technical Field

[0001] This invention relates to the field of advanced high-strength steel materials and their preparation technology, and particularly to a cold-rolled TRIP780 steel containing vanadium-titanium microalloys and its production method. It is suitable for manufacturing key structural components for new energy vehicles, and housings for energy storage devices. Background Technology

[0002] TRIP780 steel, due to its transformation-induced plasticity effect, is widely used in automotive lightweighting. However, existing technologies face three major bottlenecks: First, traditional processes rely on high-temperature hot rolling (1100-1250℃) and expensive alloys (niobium and molybdenum additions of 0.08-0.12%), resulting in energy consumption exceeding 650 kWh per ton of steel and alloy costs accounting for 12%. Second, the insufficiency of residual austenite stability leads to a strength-ductility product mostly between 22-25 GPa·%, which is insufficient to meet the performance requirements of new energy vehicle power battery trays (requiring ≥26 GPa·%). Third, the use of mineral-based rolling oil during rolling results in a biodegradation rate of <30%, leading to high environmental compliance costs. While vanadium-titanium microalloying has applications, it suffers from technical drawbacks: vanadium alone easily forms coarse VC precipitates (size >100 nm), leading to decreased toughness; titanium addition exceeding 0.06% generates TiC, consuming dissolved carbon and weakening the TRIP effect. Meanwhile, low-temperature rolling (<900℃) faces the problems of a surge in deformation resistance (30-40% higher than conventional processes) and difficulty in removing surface iron oxide scale, which restricts its industrial application.

[0003] Existing technology 201010291488.X, "High-strength Cold-Rolled Transformation-Induced Plasticity Steel Sheet and Its Preparation Method," relies on relatively expensive alloying elements such as Cu (0.1-1.0%) and Ni (0.1-1.0%) for strengthening, without mentioning a low-cost microalloying synergistic scheme. This easily drives up the alloy cost per ton of steel. The hot rolling heating temperature needs to be 1100-1250℃, which is in the high-temperature range, resulting in significant process energy consumption. There are no technical solutions to reduce energy consumption in the heating and rolling stages, which does not conform to the trend of low-energy production. Existing technology 201810290120.8, "A Cold-Rolled Transformation-Induced Plasticity Steel and Its Preparation Method," has a hot rolling heating temperature of 1220-1300℃ and a final rolling temperature of 880-940℃, resulting in high energy consumption throughout the process. Furthermore, it does not mention grain refinement measures, does not specify the grain size level, and does not mention the type of rolling oil or waste liquid treatment scheme, defaulting to the use of traditional mineral-based rolling oil (with low biodegradability). The existing technology 201410174042.7, "A Vanadium Microalloyed Hot-Rolled Transformation-Induced Plasticity Steel and Its Preparation Method", only uses vanadium single microalloying (V 0.16-0.20%), without introducing titanium elements, and cannot form VC-TiN composite precipitates. In addition, it does not optimize the heating-rolling temperature coupling mechanism, and cannot reduce energy consumption through "dynamic recrystallization-precipitation strengthening" coupling control, which is not in line with the trend of green steel production.

[0004] Therefore, developing a cold-rolled TRIP780 steel containing vanadium-titanium microalloys and its production method has become a technical problem that urgently needs to be solved in this field. Summary of the Invention

[0005] This invention aims to develop a cold-rolled TRIP780 steel containing vanadium-titanium microalloys and its production method. It develops a vanadium-titanium synergistic microalloying system to form nanoscale VC-TiN composite precipitates with vanadium and titanium. Through solid solution regulation of carbon and grain refinement, the stability of residual austenite is improved, achieving a strength-ductility product ≥26 GPa・%. Low furnace exit temperature, low-temperature coiling, and optimized hot rolling heating regime are adopted to improve the dissolution effect of microalloying elements, reduce energy consumption, and simultaneously solve the problems of deformation resistance and oxide scale control.

[0006] To achieve the above objectives, the present invention employs the following technical solution: A cold-rolled TRIP780 steel containing vanadium-titanium microalloys has the following chemical composition by weight percentage: C: 0.18%–0.22%, Si: 0.8%–1.2%, Mn: 1.6%–2.0%, V: 0.02%–0.08%, P: ≤0.015%, S≤0.005%, Ti: 0.01%–0.05%, Als: 0.02%–0.05%, N≤0.006%, with V / Ti = 2–4, Ti / N ≥ 3.42, and Mn / C = 8–10; the balance being Fe and unavoidable impurities.

[0007] Tensile strength 780-900 MPa, yield strength 450-550 MPa, elongation 30%-35%, strength-ductility product 26-32 GPa.

[0008] The microstructure of the steel plate consists of ferrite and austenite (MA) with an average grain size of 3–5 μm and a residual austenite content of 12%–15% (detected by XRD).

[0009] The highest hardness of the heat-affected zone of the welded steel plate is 310HV, and the impact energy at -40℃ is ≥70J.

[0010] A method for producing cold-rolled TRIP780 steel containing vanadium-titanium microalloys includes hot metal pretreatment, steelmaking and continuous casting, hot rolling, pickling rolling, continuous annealing and leveling; specifically including: 1) Hot metal pretreatment (50tKR desulfurization unit): Pretreatment target: S≤0.003%, total inclusions≤20mg / 100g molten steel. Process steps: Pour molten iron into the desulfurization ladle at 1250~1300℃, and take samples to analyze the initial S content; add CaO~MgO composite powder (CaO:MgO=3:1) at a rate of 1.5~2.0kg / t of molten iron, and stir through a φ300mm stirring head at a stirring speed of 120~150r / min for 8~12min; let stand for 3~5min, remove the desulfurization slag (thickness ≥100mm), take samples again to test the S content, and tap the iron after it meets the standard.

[0011] Key equipment: KR stirring device: power 160kW; online sulfur analyzer: detection accuracy 0.0001%. 2) Steelmaking continuous casting process (150t converter + LF refining + slab continuous casting machine): Converter smelting: 120t of molten iron and 30t of scrap steel are charged, using top and bottom blowing, with an oxygen supply intensity of 3.0-3.5m. 3 / (t・min); Endpoint control: C=0.06%~0.08%, temperature 1650~1680℃, P≤0.012% at tapping; Add silicon manganese alloy (10~12)kg / t, ferrovanadium (0.2~0.8)kg / t, and ferrotitanium (0.1~0.5)kg / t during tapping, and alloying time ≥3min. LF refining: producing white slag (CaO~Al2O3~SiO2 system, basicity 3.0~4.0), refining temperature 1580~1620℃. Calcium treatment: feeding Ca~Si wire (Ca content 30%), feeding speed 2~3m / s, addition amount 0.2~0.3kg / t, to spheroidize inclusions (roundness ≥0.8).

[0012] Vacuum degassing: vacuum degree ≤67Pa, processing time 15~20min, so that [H]≤2ppm. Continuous casting process: Slab dimensions: 220mm (thickness) × 1500mm (width) × 9000mm (length); Crystallizer: Copper plate material (Cr~Zr~Cu), taper 0.6~0.8% / m, cooling water flow rate 300~350m³ / m. 3 / h; Electromagnetic stirring: crystallizer stirring (current 300~350A, frequency 2~3Hz) + secondary cooling section stirring (current 250~300A, frequency 1~2Hz); Cooling regime: secondary cooling water ratio 0.8~1.0L / kg, segmented cooling (front section 30%, middle section 50%, rear section 20%); Continuous casting speed: 1.2~1.5m / min. 3) Hot rolling includes: Furnace control: The slab heating adopts a walking beam furnace with three heating stages: preheating stage, heating stage, and soaking stage. Temperature regime: Preheating stage temperature 600-700℃, heating rate from preheating stage to heating stage (8-12℃ / min), reducing the dwell time in the low-temperature stage; Heating stage adopts stepped heating, heating stage I (1000-1050℃), heating stage II (1100-1150℃), avoiding excessive temperature difference between the inside and outside of the slab and ensuring uniform dissolution of microalloys; Soaking stage temperature 1160-1200℃, holding time 60-90min. Slab exit temperature 1160-1200℃; By reducing the exit temperature, the formation and thickening of iron oxide scale are suppressed, reducing the total thickness of iron oxide scale by 20%-35% compared to the original process with an exit temperature of 1220-1250℃, while also changing its structural composition and adhesion properties.

[0013] Burner control: air-fuel ratio 1.1-1.2, ensuring no oxidative heating, with an oxidation burn-off rate ≤0.5%. Roughing process: Four-high reversible mill, roll diameter φ1200mm, material Cr5 forged steel; rolling passes: 5 passes (exit thickness 20-25mm) roll gap lubrication (oil-water emulsion, concentration 3%-5%). Finishing rolling process: seven-stand continuous rolling mill (F1~F7), work roll diameter φ600~700mm (F1~F4), φ500~600mm (F5~F7); temperature control: final rolling temperature is 800~830℃; exit thickness is 3.0~5.0mm, total reduction rate is 80~85%. Rolling speed: F1 (3m / s) → F7 (12m / s), acceleration 0.5m / s² 2 . Laminar flow cooling: front-end cooling (30-50m from the winding machine), water ratio of upper and lower manifolds 1:1.2, cooling rate 20-30℃ / s, winding temperature 530-570℃. Winding: Underground winding machine (φ1200mm drum), tension control: head 50~60kN → stable section 80~90kN → tail 60~70kN, coil ellipticity ≤3mm.

[0014] The selection criteria for the hot rolling final rolling temperature of 800-830℃ are as follows: This temperature range is in the non-recrystallized region of austenite, and TiN particles (size 20-30nm) can be precipitated by strain-induced precipitation, which can inhibit the coarsening of austenite grains; if the temperature is >850℃, TiN particles dissolve, the grains coarse to more than 6μm, and the strength-ductility product decreases by 15%.

[0015] 4) Pickling and rolling process (push-pull pickling + six-roll cold rolling mill): Pickling process: Pickling tank parameters: 4 tanks in series, tank length 15m, HCl concentration: 1# tank 10~12%, 2# tank 14~16%, 3# tank 18~20%, 4# tank 16~18%. Temperature control: Steam heating is used at 80-85℃, strip running speed is 60-80m / min, and pickling time is 5-6min.

[0016] Brushing: Two brush rollers (φ500mm, speed 300r / min) are used to remove residual iron salts from the surface. Rinsing: Three-stage countercurrent rinsing, conductivity ≤50μS / cm, drying temperature 120~150℃. Cold rolling process: Six-roll CVC mill, work roll material VC roll (chrome plating thickness 100~120μm), roll diameter φ450mm.

[0017] Rolling parameters: Inlet thickness 3.0 mm, outlet thickness 0.7–0.8 mm. Rolling oil: High-lubricity rolling oil with a dynamic friction coefficient ≤0.15 is used for cold rolling. Specifically, plant-based ester oil (mainly rapeseed oil methyl ester) can be used, with a concentration of 4%–6%, an oil supply temperature of 40–45℃, a rolling force of 2500–3000 kN, and a total reduction rate of 60%–70%. Tension control: Unwinding tension 10~15kN → Rolling tension 30~40kN → Winding tension 20~25kN, speed 1200~1500m / min.

[0018] 5) Continuous annealing and leveling process (continuous annealing furnace + four-roll leveler): Temperature control is divided into four sections; soaking zone temperature: 780–800℃, holding time 90–140s; slow cooling zone final temperature: 680–700℃; rapid cooling zone final temperature: 400–440℃, cooling rate 25–35℃ / s; over-aging zone: 380–420℃, time 300–420s. Protective atmosphere: N2 + H2, where H2 volume percentage is 5%–10%, dew point ≤40℃, furnace pressure 20–30Pa. Strip speed: 80–100m / min, plate temperature uniformity ±5℃. Leveling process: Four-roll leveler, working roll diameter φ600mm, surface roughness Ra0.8~1.0μm; parameters: elongation 0.2%~0.4%, rolling force 2000~4000kN, speed 800~1000m / min. Finishing solution: Alkaline emulsion (pH8~9), concentration 2%~3%, used to reduce the coefficient of friction, with the coefficient of friction controlled at 0.1~0.12.

[0019] This invention develops a coupled control technology of "low-temperature rolling-induced dynamic recrystallization-TiN / VC composite precipitation strengthening". Dynamic recrystallization is induced by final rolling at 800-830℃ (in the non-recrystallized austenite region), while TiN / VC composite particles are precipitated by strain, achieving synergistic grain refinement and matrix strengthening. High-lubricity rolling oil (dynamic friction coefficient ≤0.15) is used to reduce the cold rolling deformation resistance by 15-20% and control the surface roughness at Ra≤0.8μm.

[0020] The rationale for the chemical composition design of this invention is as follows: Carbon (C): 0.18%–0.22%, Function: Solid solution strengthening of austenite, ensuring residual austenite content (≥10%). Control criteria: <0.18% results in insufficient stability of residual austenite; >0.22% easily forms network carbides, reducing weldability.

[0021] Silicon (Si): 0.8%–1.2%, Function: Inhibits carbide precipitation in ferrite and promotes the TRIP effect. Control criteria: <0.8% has a weak inhibitory effect; >1.2% leads to severe oxidation of the hot-rolled surface. Manganese (Mn): 1.6%–2.0%, Function: Expands the austenite region and improves hardenability. Control basis: Forms composite alloys with C; controlling the Mn / C ratio at 8–10 can optimize phase transformation kinetics. Vanadium (V): 0.02%–0.08%, Function: Forms VC nanoprecipitates (20-50 nm), a dislocation pinning and strengthening material. Control basis: Synergistic with Ti, the most uniform precipitate distribution is achieved when the V / Ti ratio is 2–4. Titanium (Ti): 0.01%~0.05%, Function: Preferentially combines with N to form TiN (10-30nm), refining austenite grains (average size ≤5μm). Control basis: A Ti / N ratio ≥3.42 (atomic ratio) can completely fix N. Aluminum (Al): 0.02%–0.05%, Function: Deoxidation and inhibition of carbide precipitation. Control basis: Als (acid-soluble aluminum) must be ≥0.02% to ensure deoxidation effect.

[0022] Impurity elements: P≤0.015% (to avoid cold brittleness), S≤0.005% (to reduce MnS inclusions), N≤0.006% (to prevent porosity).

[0023] When V / Ti = 3, Ti preferentially forms TiN with N (pinning grain boundaries), and the remaining V forms VC with C (strengthening the grain interior), with the composite precipitate density reaching 5 × 10⁻⁶. 15 pcs / m 3 The strength contribution is maximized; if V / Ti>4, coarse VC (>100nm) is easily formed, resulting in the elongation dropping to below 25%.

[0024] This invention forms VC-TiN composite precipitates (20-50nm) by adjusting the V / Ti ratio to 2-4. This not only utilizes TiN to refine the grains (average grain size 3-5μm) but also strengthens the matrix by VC to improve strength, thus solving the performance shortcomings of single microalloying.

[0025] Compared with the prior art, the beneficial effects of the present invention are: 1. This invention develops a vanadium-titanium synergistic microalloying system, enabling vanadium and titanium to form nanoscale VC-TiN composite precipitates (20-50nm). Through solid solution regulation of carbon and grain refinement, the stability of retained austenite is improved, stabilizing the retained austenite content in the steel plate microstructure at 12%–15%, achieving a strength-ductility product ≥26GPa・; lower furnace exit temperature reduces the thickness of the iron oxide scale, decreasing the furnace exit temperature from 1220-1260℃ to 1160-1200℃, reducing the total iron oxide scale thickness by 20%-35%; optimizes the hot rolling heating regime, improving the dissolution effect of microalloying elements, and adopts a low-temperature coiling process to thin the Si-rich layer of the iron oxide scale, decreasing the coiling temperature from 630℃ to 550℃, reducing the Si-rich layer thickness by 30%-35%.

[0026] 2. Tested according to GB / T228.1-2021 "Metallic Materials - Tensile Testing - Part 1: Test Method at Room Temperature", the specimen type is a plate specimen (gauge length 50mm). Mechanical properties: tensile strength 780-900MPa, yield strength 450-550MPa, elongation 30-35%, strength-ductility product 26-32GPa・%, superior to existing TRIP780 steel (strength-ductility product 22-25GPa・%); microstructure: retained austenite content 12-15% (8-10% in traditional process), average grain size 3-5μm (8-12μm in traditional process). The highest hardness of the welded heat-affected zone of the steel bar is 310HV (350HV in traditional process), and the impact energy at -40℃ is ≥70J, meeting the low-temperature service requirements of new energy vehicles.

[0027] 3. Hot rolling heating energy consumption is reduced from 320 kW·h / t to 240 kW·h / t, resulting in a 22% reduction in overall energy consumption throughout the process (saving 44,000 tons of standard coal for an annual production capacity of 500,000 tons). The cost of vanadium-titanium alloy (0.15-0.2 yuan / kg) is 75-85% lower than that of niobium-molybdenum (0.8-1.0 yuan / kg), resulting in a saving of 120-150 yuan per ton of steel alloy cost. Attached Figure Description

[0028] Figure 1 This is a 500x metallographic microstructure image of the sample from Example 2. Detailed Implementation

[0029] 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.

[0030] This invention relates to a cold-rolled TRIP780 steel containing vanadium-titanium microalloys and its low-temperature rolling production process, aiming to improve the material's strength and toughness, reduce production energy consumption, and improve environmental performance through composition optimization and process innovation.

[0031] The chemical composition and mass percentage of the examples are shown in Table 1; the smelting process of the examples is shown in Table 2; the hot rolling process of the examples is shown in Table 3; the pickling and cold rolling process parameters of the examples are shown in Table 4; the continuous annealing process of the examples is shown in Table 5; and the mechanical properties and microstructure of the examples are shown in Table 6.

[0032] Example 1 is a steel for a power battery tray in a new energy vehicle, used in a power battery tray (1800×1200×150mm) of a pure electric SUV. Through lightweight design (thickness reduced by 0.2mm compared to traditional steel), it achieves a 15% weight reduction and exhibits no cracking at a bending angle of 180° (d=2a). Example 2 is a steel for a drive motor housing in a new energy vehicle, used in the housing of an 800V high-voltage motor (diameter 350mm, height 200mm). High-precision stamping (tolerance ±0.1mm) meets the motor's sealing requirements and exhibits excellent resistance to hydrogen embrittlement (hydrogen permeation current density 0.08μA / cm²). 2 It meets the IP6K9K protection standard, with a welded joint strength of 850MPa (heat input 1.5kJ / mm). Example 3 is the steel used for the frame of an energy storage container, which is used for the load-bearing frame (section size 100×50mm) of a 100MWh energy storage container. The frame is lightweighted by welding assembly (welding efficiency improved by 20%), meets the weather resistance requirements in the high humidity environment of the seaside, and has a salt spray resistance (5000h) corrosion level ≤1. Table 1 Chemical composition of the examples (%): Table 2 Smelting process parameters for the examples: Table 3 Hot rolling process parameters for the examples: Table 4. Pickling and cold rolling process parameters for the examples: Table 5 Examples of continuous annealing processes: Table 6 Mechanical properties and microstructure of the embodiments: The microstructure of the sample exhibits a typical dual-phase dominant structure of ferrite (F) and mausoleum (MA), with localized small amounts of bainitic ferrite (B), and no precipitation of brittle phases such as pearlite (P) or Widmanstätten (W), which meets the design goal of "inhibiting carbide precipitation and optimizing the TRIP effect." Figure 1 As shown, the metallographic microstructure of the sample of Example 2 of the present invention is magnified 500 times. Table 7 shows the corresponding phase composition and grain size information. The average grain size level of the sample is grade 12 (GB / T6394-2017), and the corresponding average grain size is about 4.4 μm, which is 50-65% finer than traditional TRIP780 steel (grade 8-10, grain size 8-12 μm). Moreover, the standard deviation of grain size is ≤0.3 μm, which shows excellent microstructure uniformity.

[0033] Table 7 shows the grain size, phase composition, and other information corresponding to the metallographic structure of Example 2: The core mechanism of grain refinement in Example 2 stems from the synergistic effect of "low-temperature rolling + TiN pinning" in this invention: the hot rolling final rolling temperature of 810℃ (40-80℃ lower than the traditional process) is in the non-recrystallized austenite region. Through strain-induced dynamic recrystallization (SIDR), a large number of dislocation cells are formed in the austenite grains, providing sites for grain nucleation during the subsequent annealing process; Ti elements preferentially combine with N to form TiN nanoparticles (10-30nm), which have excellent high-temperature stability (melting point 2950℃) and do not dissolve during heating and final rolling. Through the Zener pinning effect, they hinder the growth of austenite grains, laying the foundation for the fine-grained structure after cold rolling and annealing; the continuous annealing homogenization temperature of 780℃ (critical zone annealing) avoids excessive growth of ferrite grains, while promoting the nucleation of VC precipitates at grain boundaries and dislocations, further refining the ferrite grains to about 4.5μm. Fine-grained microstructure not only improves plasticity but also enhances the welding performance of motor housings—the degree of grain coarsening in the heat-affected zone (HAZ) is reduced, and the maximum hardness is controlled at 310HV (350HV in traditional processes), avoiding the risk of cold cracking during welding.

[0034] According to GB / T10561-2023 "Standard Rating Chart Method", the non-metallic inclusions of the sample are rated as DT1.0 (point-like non-deformable inclusions), with no long strip-shaped sulfides (Class A), alumina (Class B) or silicates (Class C) inclusions, and the maximum size of a single inclusion is ≤5μm, and the total inclusion area fraction is ≤0.05% (approximately 0.1-0.15% in traditional processes). The inclusion control effect stems from the patented "full-process cleanliness control" technology: In the molten iron pretreatment stage, KR stirring desulfurization (CaO-MgO composite powder, CaO:MgO=3:1) is used to reduce the S content to 0.002%, thus reducing the formation of MnS inclusions; in the LF refining stage, white slag (CaO-Al2O3-SiO2 system, basicity 3.0-4.0) is used to adsorb alumina inclusions, and Ca-Si wire (Ca content 30%) is fed in to spheroidize residual inclusions (roundness ≥0.8), preventing angular inclusions from becoming stress concentration sources; in the continuous casting stage, a weak cooling regime (specific water volume 0.8-1.0 L / kg) and electromagnetic stirring (crystallizer + secondary cooling section) are used to reduce the agglomeration of inclusions in the center of the billet. The low inclusion content significantly improves the sample's resistance to hydrogen embrittlement, with a hydrogen permeation current density ≤0.08 μA / cm². 2 (Traditional process yields approximately 0.15 μA / cm) 2 It meets the requirements for hydrogen embrittlement resistance in contact between the motor housing and the battery system (IP6K9K protection standard).

[0035] The correlation between microstructure and application performance: The microstructure characteristics of Example 2 directly support its core performance requirements as steel for the housing of drive motors in new energy vehicles, demonstrating the practicality and innovation of this invention: High strength and resistance to deformation: The tensile strength of 910 MPa originates from the synergistic effect of "fine grain strengthening (contributing 30%) + VC precipitation strengthening (contributing 35%) + MA component strengthening (contributing 25%) + residual austenite TRIP effect (contributing 10%)", ensuring that the deformation rate of the motor housing is ≤0.1% (compared to approximately 0.3% for traditional steel) when subjected to the vibration load of an 800V high-voltage motor, guaranteeing a tight seal. Sealing performance; resistance to hydrogen embrittlement and corrosion: low inclusion content (DT1.0 grade) reduces hydrogen trap sites, and high-carbon residual austenite hinders hydrogen diffusion, significantly reducing the sensitivity of the sample to hydrogen-induced cracking during motor housing welding and service, meeting the service requirements of the hydrogen environment around the power battery system; precision formability: fine grain structure and uniformly distributed MA components (no agglomeration) result in a short yield plateau (yield elongation ≤1.5%) during stamping, with plate thickness difference controlled within ±0.05mm (conventional steel ±0.1mm), meeting the forming tolerance requirements (±0.1mm) of the high-precision sealing structure of the motor housing.

[0036] The hydrogen permeation current density of all embodiments was tested according to GB / T24186-2022 "Electrochemical Method for Hydrogen Permeation Test of Metallic Materials", and the salt spray resistance was performed according to GB / T10125-2021 Neutral Salt Spray Test (5% NaCl solution, temperature 35℃).

Claims

1. A cold-rolled TRIP780 steel containing vanadium-titanium microalloys, characterized in that, The chemical composition of the steel, by weight percentage, is as follows: C: 0.18%~0.22%, Si: 0.8%~1.2%, Mn: 1.6%~2.0%, V: 0.02%~0.08%, P: ≤0.015%, S≤0.005%, Ti: 0.01%~0.05%, Als: 0.02%~0.05%, N≤0.006%, and V / Ti=2~4, Ti / N≥3.42, Mn / C=8~10; the balance is Fe and unavoidable impurities.

2. The cold-rolled TRIP780 steel containing vanadium-titanium microalloying according to claim 1, characterized in that, Tensile strength 780-900 MPa, yield strength 450-550 MPa, elongation 30%-35%, strength-ductility product 26-32 GPa.

3. The cold-rolled TRIP780 steel containing vanadium-titanium microalloying according to claim 1, characterized in that, The microstructure of the steel plate consists of ferrite and mausoleum, with an average grain size of 3-5 μm and a retained austenite content of 12%-15%.

4. The cold-rolled TRIP780 steel containing vanadium-titanium microalloying according to claim 1, characterized in that, The highest hardness of the heat-affected zone of the welded steel plate is 310HV, and the impact energy at -40℃ is ≥70J.

5. A method for producing cold-rolled TRIP780 steel containing vanadium-titanium microalloys as described in any one of claims 1-4, comprising hot metal pretreatment, steelmaking and continuous casting, hot rolling, pickling rolling, continuous annealing, and leveling; characterized in that, The hot rolling temperature regime is as follows: preheating zone temperature: 600-700℃, heating rate from preheating zone to heating zone: 8-12℃ / min, heating zone I: 1000-1050℃, heating zone II: 1100-1150℃, soaking zone temperature: 1160-1200℃, soaking zone holding time: 60-90min; slab exit temperature: 1160-1200℃, final rolling temperature: 800-830℃, coiling temperature: 530-570℃.

6. The method for producing cold-rolled TRIP780 steel containing vanadium-titanium microalloys according to claim 5, characterized in that, In the molten iron pretreatment process, S ≤ 0.003% and total inclusions ≤ 20 mg / 100g molten steel are controlled.

7. The method for producing cold-rolled TRIP780 steel containing vanadium-titanium microalloys according to claim 5, characterized in that, In the steelmaking and continuous casting process: the converter smelting endpoint is controlled at C=0.06%~0.08%, temperature 1650~1680℃, and P≤0.012% at tapping; LF refining produces white slag and calcium treatment to make the spheroidization and roundness of inclusions ≥0.8; Continuous casting cooling water flow rate 300-350m³ 3 / h; the electromagnetic stirring current of the crystallizer is 300-350A, and the frequency is 2-3Hz; the electromagnetic stirring current of the secondary cooling section is 250-300A, and the frequency is 1-2Hz; the secondary cooling specific water volume is 0.8-1.0L / kg, and the continuous casting speed is 1.2-1.5m / min.

8. The method for producing cold-rolled TRIP780 steel containing vanadium-titanium microalloys according to claim 5, characterized in that, In the pickling and rolling process, four pickling tanks are connected in series. The HCl concentrations are: 10%–12% for tank #1, 14%–16% for tank #2, 18%–20% for tank #3, and 16%–18% for tank #4. The pickling temperature is 80–85℃, the strip running speed is 60–80 m / min, and the pickling time is 5–6 min. The coefficient of dynamic friction of cold rolling oil is ≤0.15, the oil supply temperature is 40~45℃, the rolling force is 2500~3000kN, and the total reduction rate is 60%~70%.

9. The method for producing cold-rolled TRIP780 steel containing vanadium-titanium microalloys according to claim 5, characterized in that, The continuous annealing process is divided into four temperature control stages: the temperature of the heat soaking stage is 780-800℃, and the holding time is 90-140s; the final temperature of the slow cooling stage is 680-700℃; the final temperature of the rapid cooling stage is 400-440℃, and the cooling rate is 25-35℃ / s; the temperature of the over-aging stage is 380-420℃, and the time is 300-420s; the dew point is ≤40℃, and the furnace pressure is 20-30Pa.

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