An ultra-high strength pickling-free hot-dip galvanized steel for automobiles and a production method thereof

Abstract

This invention belongs to the field of metallic materials, specifically relating to an ultra-high strength, acid-free, hot-dip galvanized automotive steel and its production method. The steel sheet of this invention is mainly suitable for manufacturing automotive structural parts. The chemical composition of the steel is: C, Si, Mn, Al, Nb, Ti, Cr, Mo, W, Cu, Zr, Ce, Ge, Pr, with C / Nb: 1.50–2.50, C / Ti: 0.70–1.14, Mo+Cr: 0.85%–1.00%, and the balance being Fe and unavoidable impurities. The steel sheet of this invention has a yield strength ≥985 MPa, tensile strength ≥1185 MPa, longitudinal elongation A ≥25.5%, and meets the acceptable transverse cold bending standard (D=a) at 180°. It exhibits good surface quality and is free of color difference defects. The steel is obtained through a hot-rolling + reduction descaling + continuous annealing galvanizing + finishing process, reducing the need for pickling and cold rolling, resulting in low production costs and no environmental pollution.

Description

Technical Field

[0001] This invention belongs to the field of metallic materials, and specifically relates to an ultra-high strength, acid-free, hot-dip galvanized automotive steel and its production method. The steel sheet of this invention is mainly suitable for manufacturing automotive structural components. Background Technology

[0002] The automotive, steel, petrochemical, and construction industries are collectively known as the four pillars of the national economy. Among them, the scale of the automotive and steel industries is a core indicator of a country's industrial development level, and major industrial powers worldwide have prioritized the automotive industry as a pillar sector of their national economies. Simultaneously, the automotive industry is a core application area for steel products, and the research, development, and application of steel products are deeply intertwined with the development trends of the automotive industry. Currently, my country's automotive industry is entering a stage of rapid development, and automotive steel is upgrading towards lower costs, higher strength, green environmental protection, and higher safety. To reduce the weight of automotive structural components and achieve energy conservation, emission reduction, and environmental protection goals, the application scenarios of high-performance, high-surface-quality automotive steel sheets are continuously expanding, and their adoption is accelerating. Currently, the production of most automotive galvanized steel sheets requires pickling and cold rolling, which not only pollutes the environment but also incurs high costs for waste acid treatment, and the cold rolling process also increases production costs.

[0003] Chinese patent application CN 121320821 A discloses a production method for ultra-high strength galvanized duplex steel strip with a high yield strength of 1000MPa, including: smelting-continuous casting process, hot rolling process, pickling and cold rolling process, continuous hot-dip galvanizing process, and finishing process. Its chemical composition by mass percentage is: C: 0.11-0.15%, Si: ≤0.40%, Mn: 2.60-2.85%, P ≤0.020%, S ≤0.010%, Alt: 0.015-0.060%, Cr: 0.10-0.30%, Nb: 0.025-0.040%, Ti: 0.010-0.025%, Ca: 0.0008-0.0020%, N ≤0.0070%, with the remainder being Fe and impurities. Yield strength Rp 0.2 733~752MPa, tensile strength R m 1059~1109MPa, elongation A 50 The elongation is 13.0% to 16.0%. Its elongation does not meet the production requirements of ultra-high strength automotive parts. Furthermore, the manufacturing process involves hot rolling + pickling + cold rolling + continuous hot-dip galvanizing + finishing, which involves a large process flow and high production costs. In particular, pickling is required, which results in significant acid mist pollution, environmental pollution, and high waste acid treatment costs.

[0004] Chinese patent application CN 116607081 A discloses a 980MPa high-strength automotive steel sheet and its production method, including smelting, continuous casting, hot rolling, leveling, coiling, pickling and cold rolling processes; and the process is prepared by hot rolling + pickling + cold rolling + continuous annealing + finishing process, which has a large process flow and high production cost. In particular, pickling is required, which causes a lot of acid mist pollution, pollutes the environment, and has high waste acid treatment costs. Summary of the Invention

[0005] To address the evolving needs of the automotive steel industry, this invention presents an ultra-high strength, pickle-free, hot-dip galvanized automotive steel and its production method. The steel sheet exhibits a yield strength ≥985MPa, tensile strength ≥1185MPa, longitudinal elongation A ≥25.5%, and meets the acceptable transverse cold bending standard (D=a at 180°). It boasts excellent surface quality, free of color defects, and a surface roughness Ra of 0.90-1.60μm. Furthermore, it eliminates the need for conventional hot rolling + pickling + cold rolling + continuous annealing galvanizing + finishing processes. Instead, it employs a hot rolling + reduction descaling + continuous annealing galvanizing + finishing process, reducing the pickling and cold rolling steps, resulting in lower production costs and no environmental pollution.

[0006] To achieve the above objectives, the present invention employs the following technical solution: A type of ultra-high strength, acid-free, hot-dip galvanized automotive steel, with the following chemical composition by weight percentage: C: 0.102%–0.162%, Si: 0.15%–0.30%, Mn: 2.00%–2.22%, Al: 0.035%–0.056%, Nb: 0.053%–0.072%, Ti: 0.130%–0.145%, Cr: 0.50%–0.60%, Mo: 0.30%–0.45%, W: 0.015%–0.042%, Cu The composition of Fe is 0.16%–0.22%, Zr is 0.015%–0.025%, Ce is 0.0035%–0.0082%, Ge is 0.0055%–0.012%, Pr is 0.016%–0.022%, and C / Nb is 1.50–2.50, C / Ti is 0.70–1.14, Mo+Cr is 0.85%–1.00%, with P ≤ 0.012%, S ≤ 0.005%, N ≤ 0.006%, and the balance being Fe and unavoidable impurities.

[0007] The finished steel plate has a ferrite volume percentage of 2%–10%, a bainite volume percentage of 10%–20%, a martensite volume percentage of 60%–70%, and a retained austenite volume percentage of 5%–10%.

[0008] The finished steel plate has a yield strength ≥985MPa, tensile strength ≥1185MPa, longitudinal elongation A ≥25.5%, transverse cold bending 180° D=a qualified, surface roughness Ra is 0.90~1.60μm, and the thickness of the finished steel plate is 1.62~1.92mm.

[0009] The main function of the ultra-high strength, acid-free, hot-dip galvanized automotive steel composition in this invention is as follows: C: Carbon is a core strengthening element in steel. As an interstitial solid solution atom, it distorts the matrix lattice, achieving solid solution strengthening. In this invention, the key role of carbon is to ensure that the bainite and martensite content meets the standards, thereby simultaneously improving the strength and formability of the steel plate. If the carbon content is too low, the steel plate cannot meet the mechanical performance indicators required by this invention; if the content is too high, it will lead to embrittlement of the steel plate, causing delayed fracture and hot-rolling edge cracking, while significantly deteriorating its weldability and toughness. Therefore, this invention controls the carbon content within a low carbon range, which can avoid the risks of delayed fracture and hot-rolling edge cracking, and also improve the weldability of the steel plate. Therefore, the optimal range of carbon in this invention is 0.102-0.162%.

[0010] Si: Silicon is the core element of this invention. Sufficient silicon addition to ferrite ensures the strength of the ferrite matrix and increases the AC3 temperature of the steel plate, widening the continuous annealing process window and ensuring that the ratio of ferrite to austenite in the critical region is within a reasonable range at industrial continuous annealing temperatures. Simultaneously, sufficient silicon content reduces inclusions in the steel and inhibits their formation, preventing inclusions from causing a decline in the mechanical properties of the steel plate. However, the silicon content must be controlled within a reasonable range: too low a content will fail to guarantee the matrix strength and achieve the inclusion inhibition effect, while too high a content will damage the surface quality of hot-rolled steel, causing a large amount of iron oxide scale to form and deteriorating the weldability of the steel plate. Therefore, the silicon content in this invention is 0.15-0.30%.

[0011] Mn: Manganese introduces lattice distortion into steel through substitutional solid solution, achieving solid solution strengthening. Simultaneously, as an austenite stabilizing element, it can widen the austenite region, reduce the critical quenching rate, and delay the transformation of austenite to pearlite. However, insufficient manganese content reduces the stability of supercooled austenite, leading to decreased plasticity and toughness of the steel plate. Manganese can dissolve in both ferrite and austenite phases, significantly improving the strength and hardness of both phases. The manganese content must be strictly controlled within a set range; excessive amounts can easily induce carbon or manganese segregation, disrupting the uniformity of the microstructure during hot rolling and inducing severe banded defects. It can also excessively increase the hardenability of the steel, inhibit bainite formation, impair the mechanical properties of the steel plate, and significantly degrade weldability. Therefore, considering all factors, this invention selects a manganese content of 2.00-2.22%.

[0012] P: Phosphorus is a harmful impurity in steel and tends to agglomerate at grain boundaries. When the phosphorus content in steel exceeds the standard, Fe2P particles will be generated, causing a decrease in the plasticity, toughness, and formability of the steel. Therefore, the phosphorus content must be strictly controlled, and the lower the content, the better it is for improving the formability of the steel. Therefore, its upper limit is set at 0.012%.

[0013] S: Sulfur is a harmful impurity in steel. It easily reacts with manganese to form MnS inclusions. These inclusions become the core source of crack initiation, which not only deteriorates the processing performance of steel, but also significantly reduces the plasticity and formability of steel plates. Therefore, the lower the content, the better. The upper limit is set at 0.005%.

[0014] Al: In traditional steelmaking processes, Al is a deoxidizer. Al can also combine with nitrogen in steel to form AlN, refining the grain size. Simultaneously, together with Si, it inhibits cementite precipitation, increases the austenitizing temperature, facilitates better selection of the process window, and accelerates bainite transformation. Excessive Al content will cause nozzle blockage during continuous casting, affecting production efficiency and increasing production costs. Therefore, in this invention, the Al content is limited to 0.035-0.056%.

[0015] Niobium (Nb) effectively delays the recrystallization of deformed austenite, inhibits austenite grain growth, increases the austenite recrystallization temperature, and refines the grain size. Simultaneously, it improves the strength and toughness of the steel. Nb can refine the martensite in the steel of this invention, suppress banded structures, improve stamping and local forming performance, and the dissolved Nb hinders martensite recovery and recrystallization, enhancing tempering stability and ensuring no strength reduction after galvanizing. It can also refine the grain size on the substrate surface, increase the wettability of the zinc bath, resulting in a more uniform coating, reduced plating defects, and stronger interfacial bonding. Therefore, the optimal range of Nb content in this invention is between 0.053% and 0.072%, and satisfies the C / Nb ratio of 1.50 to 2.50.

[0016] Ti: Titanium can effectively delay the recrystallization of deformed austenite, prevent austenite grain growth, increase the austenite recrystallization temperature, refine grains, and improve the strength and toughness of steel. Since free N atoms in steel deteriorate the toughness of steel plates, Ti combines with impurity elements N in steel to form blocky or angular TiN. Therefore, the formation of TiN has a solidifying effect on N. However, if the N content is too high, the size of the blocky or angular TiN will be too large, which will deteriorate the performance of the steel plate and make the toughness of the weld heat-affected zone and the fatigue performance of the steel plate worse. Therefore, this invention limits N to ≤0.006% to avoid the formation of too much blocky or angular TiN. Furthermore, the purpose of Ti microalloying in this invention is to enable Ti and C to form submicron or nano-sized fine spherical TiC particles for dispersed precipitation strengthening, refining the grains, strengthening the matrix, and playing a role in grain refinement strengthening and precipitation strengthening. It can also strengthen ferrite, which is beneficial to obtaining excellent mechanical properties and fatigue properties, thereby improving the service life of steel in the manufacture of automotive structural parts. Therefore, the optimal range of Ti content in this invention is between 0.130-0.145%, and it satisfies C / Ti: 0.70-1.14.

[0017] Cr: Chromium can significantly improve the corrosion resistance of hot-dip galvanized steel sheet coating and substrate, inhibit coating corrosion diffusion, improve zinc liquid wettability, reduce coating defects, and enhance substrate strength and high temperature stability. However, excessive Cr will increase the zinc layer alloying rate and easily lead to coating embrittlement. Therefore, the optimal range of Cr content in this invention is between 0.50-0.60%.

[0018] Mo: Adding Mo to steel can delay the pearlite transformation, promote the formation of ferrite or bainite, and reduce banded structures. It improves the local formability of steel sheets during stamping, preventing stamping cracks. It also significantly improves the tempering stability of steel, inhibiting the softening of martensite and bainite during tempering and ensuring that the strength does not decrease after galvanizing. Furthermore, Mo easily forms a dense oxide film on the substrate surface, reducing the substrate's corrosion tendency, ensuring a uniform galvanized layer, delaying the penetration of corrosive media, and improving resistance to atmospheric and salt water corrosion. Therefore, the optimal range of Mo content in this invention is between 0.30-0.45%, and satisfies the Mo+Cr ratio of 0.85-1.00%.

[0019] W: Tungsten can refine grains, inhibit high-temperature oxidation, improve coating quality and substrate properties, and increase the high-temperature strength of steel plates, reduce deformation during hot-dip galvanizing, improve coating adhesion and corrosion resistance, inhibit grain boundary oxidation, reduce incomplete coating defects, and improve the uniformity and surface quality of the galvanized layer. Therefore, the optimal range of W content in this invention is between 0.015-0.042%.

[0020] Cu: Adding Cu to steel can achieve precipitation strengthening by precipitating nanoscale precipitates, which can improve the strength, toughness and formability of the steel plate. It can inhibit austenite grain growth, promote ferrite nucleation, refine grains and optimize the strength-ductility match. At the same time, it can lower the A3 temperature, expand the austenite region, help regulate the proportion of phase transformation structure, and improve the uniformity and stability of the structure. However, if the Cu content is too high, it is easy to agglomerate and precipitate at the grain boundaries, which can cause grain boundary embrittlement and hot working copper embrittlement defects. Therefore, the optimal range of Cu content in this invention is between 0.16-0.22%.

[0021] Zr: Zr forms high-melting-point, spherical ZrS, ZrO2, and ZrN with impurities such as S, O, and N, purifying grain boundaries, eliminating hot brittleness, improving hot workability, and reducing interference from impurities on the zinc-iron reaction. Zr segregates at grain boundaries and precipitates dispersed carbonitrides, strongly inhibiting the growth of austenite or ferrite grains, improving the strength and toughness of the matrix, and providing a good substrate for a uniform coating. Zr forms stable composite compounds with Si, reducing Si enrichment on the steel plate surface, inhibiting the over-catalysis of the Fe-Zn reaction by Si, avoiding problems such as excessively thick coatings, dullness, and poor adhesion, and stabilizing the coating thickness and morphology. Therefore, the optimal range of Zr content in this invention is between 0.015% and 0.025%.

[0022] Ce: Cerium has a strong affinity for harmful elements such as oxygen and sulfur in steel, and can form high-melting-point stable compounds such as Ce₂O₃ and CeS. These substances easily float and separate from molten steel, effectively reducing non-metallic inclusions and improving steel purity. For inclusions that cannot be completely removed, cerium can modify their morphology and distribution, transforming elongated sulfides that easily cause brittleness into spherical or dot-shaped cerium sulfides, mitigating the adverse effects of inclusions on the mechanical properties of steel, and significantly improving the toughness and fatigue strength of steel. Simultaneously, cerium can inhibit grain growth during heating and cooling, refining austenite or ferrite grains, thereby synergistically improving the strength and toughness of steel. Through this purification and modification effect, the plasticity and toughness of steel are improved, and the risk of cracking during hot and cold working is reduced; in addition, it can optimize the weldability and oxidation resistance of steel, extending its service life in high-temperature environments. Therefore, this invention limits the Ce content to 0.0035-0.0082%.

[0023] Ge (Ge): Germanium can improve the hot working properties of steel, reduce the tendency of steel to crack during rolling, and also optimize the weldability of steel, reducing the brittleness of weld joints and improving the consistency of the mechanical properties of weld joints. Furthermore, it can optimize the adhesion of the zinc layer, inhibit excessive thickness of the Fe-Zn brittle alloy layer, reduce coating peeling, flaking, and zinc loss during stamping and bending. It can also refine the coating structure, reduce porosity, make the zinc layer more uniform and dense, improve corrosion resistance, and improve the surface condition of the substrate, reduce oxidation defects, improve the wettability of zinc bath, avoid defects such as incomplete plating and pinholes, and ensure the protective effect and appearance smoothness of the coating. Therefore, the optimal range of Ge content in this invention is between 0.0055% and 0.012%.

[0024] Praseodymium (Pr) can remove harmful impurities such as O and S from steel, spheroidize brittle inclusions in the form of strips or angular shapes, reduce interface defects caused by inclusions during hot-dip galvanizing, improve zinc layer adhesion and stamping crack resistance, reduce the risk of coating peeling, refine substrate grains and improve surface oxide layer condition, inhibit abnormal thickening of Fe-Zn intermetallic compound layer during hot-dip galvanizing, promote coating microstructure refinement, reduce porosity and zinc flower size, and increase resistance to atmospheric corrosion and pitting corrosion. Therefore, the Pr content in this invention is limited to 0.016-0.022%.

[0025] N: For the N content in steel, the lower the N content, the better. However, too low a content will lead to production difficulties and increased costs. However, this invention requires the precipitation of TiN formed with Ti to carry out precipitation strengthening and grain refinement strengthening, thereby improving the strength of the steel plate. Therefore, the N content in this invention is ≤0.006%.

[0026] A production method for ultra-high strength, acid-free hot-dip galvanized automotive steel includes smelting, hot rolling, reduction descaling, continuous annealing galvanizing, and finishing. Specific methods include: (1) Smelting process: The RH+LF process is adopted, and the H and O contents are strictly controlled. H≤0.0002% and O≤0.0015%. Calcium treatment is carried out in the refining process. Electromagnetic stirring and light reduction technology are applied in the continuous casting process. The billet casting speed is ≤1.6m / min and the light reduction is 2.0mm~2.5mm. This reduces the center segregation of the continuous casting billet, which is beneficial to reducing the banded structure in the subsequent hot-rolled steel plate.

[0027] (2) Hot rolling process: The continuous casting slab with a thickness of 220-270 mm and a width of 1020-1700 mm is directly hot-loaded into the walking beam furnace for heating at a temperature of 1230-1245℃ and a holding time of 150-200 min. The roughing process employs a 3+3 rolling mode (R1 is rolled in 3 passes, and R2 is rolled in 3 passes), for a total of 6 passes. The roughing mill exit temperature is 1090℃~1115℃, the intermediate slab thickness is 30.0~40.0mm, and the width is 1020~1700mm. The intermediate slab is insulated with a heat shield before entering the hot finishing mill to reduce temperature drop on the delay roller table and temperature differences at the beginning and end and across the width of the slab. The finishing mill is a 7-stand continuous rolling process. High-pressure water descaling is performed before finishing, with the finishing mill inlet temperature not exceeding 1090℃. The final rolling temperature is 880~945℃. After final rolling, laminar flow cooling is used at a rate of approximately 18~28℃ / s. After cooling to 630~666℃, the slab is coiled and air-cooled to room temperature. The rolled thickness is 1.62~1.92mm.

[0028] (3) Reduction Descaling: After the hot-rolled steel coil with a thickness of 1.62-1.92mm is cooled to room temperature, the iron oxide scale on the surface of the steel plate is reduced and descaled in a high-temperature continuous heating furnace. The reduction descaling principle is as follows: Formula 1-6. The main processes include uncoiling, heating reduction, surface treatment, cooling, leveling and coiling. The heating reduction atmosphere of the steel plate is a mixture of 60%-75% H2, 0%-10% CO and 20%-40% N2 by volume. The heating temperature is 680-1150℃. The surface treatment uses a heat-resistant steel wire roller brush to clean the reduced iron filings on the surface of the steel plate. The cooling gas is 100% N2 and the cooling temperature is ≤50℃. The leveling elongation is 1.1-1.6%. After reduction descaling, the volume percentage of ferrite in the steel plate is 30%-38%, the volume percentage of lamellar pearlite is 10%-64%, and the volume percentage of spheroidized pearlite is 0%-60%.

[0029] Formula 1: Fe2O3+3H2→2Fe+3H2O; Formula 2: Fe3O4+4H2→3Fe+4H2O; Formula 3: FeO+H2→Fe+H2O; Formula 4: Fe2O3+3CO→2Fe+3CO2; Formula 5: Fe3O4+4CO→3Fe+4CO2; Formula 6: FeO+CO→Fe+CO2.

[0030] (4) Continuous annealing and galvanizing: Steel coils with a thickness of 1.62-1.92 mm after reduction descaling are produced using a continuous annealing and galvanizing process. The process parameters are controlled as follows: belt speed 55-80 m / min, soaking temperature 820-840℃, holding time 10-16 min, slow cooling outlet temperature 705-720℃, rapid cooling rate greater than 45℃ / s, rapid cooling outlet temperature 450-460℃, and then fed into a zinc pot at 455-465℃ to complete hot-dip galvanizing. The galvanizing time is 1-6 s. The core of controlling the soaking temperature at 820-840℃ is to ensure that the steel coil obtains a reasonable ratio of ferrite and austenite in the critical zone. Temperatures above 840℃ will significantly reduce the proportion of ferrite, resulting in a decrease in the plasticity of the steel. Temperatures below 820℃ will result in insufficient austenite content in the critical zone, affecting the subsequent formation of bainite and martensite, ultimately leading to substandard steel strength. The holding time is set to 10–16 minutes to ensure sufficient recrystallization of the steel plate during the heating and holding stage, while avoiding excessive holding time that could lead to grain coarsening. If the time is too short, the steel plate cannot complete the continuous annealing and recrystallization process, resulting in reduced elongation. The slow cooling outlet temperature is controlled at 705–720℃ to promote rapid ferrite precipitation, which inhibits grain growth while ensuring ferrite content, thus achieving ferrite grain refinement. The rapid cooling outlet temperature is maintained at 450–460℃ to allow bainite and martensite to precipitate rapidly before the strip enters the zinc pot, inhibiting grain growth, ensuring bainite and martensite content, and achieving bainite and martensite grain refinement.

[0031] (5) Finishing: After galvanizing, the product enters the finishing machine, and the finishing elongation is 1.1 to 1.6%.

[0032] Compared with the prior art, the beneficial effects of the present invention are: 1. Niobium can effectively delay the recrystallization of deformed austenite, prevent austenite grain growth, increase the austenite recrystallization temperature, refine the grains, and improve the strength and toughness of steel. Nb can refine the martensite in the steel of this invention, suppress banded structure, improve stamping and local forming performance, and the solid-solid Nb hinders martensite recovery and recrystallization, improves tempering stability, ensures that the strength does not decrease after galvanizing, and can also refine the grains on the substrate surface, increase the wettability of zinc liquid, make the coating more uniform, reduce the rate of missed plating, and make the interface bonding stronger.

[0033] 2. Titanium can effectively delay the recrystallization of deformed austenite, prevent austenite grain growth, increase the austenite recrystallization temperature, refine the grains, and improve the strength and toughness of steel. Since free nitrogen atoms in steel deteriorate the toughness of the steel plate, Ti combines with impurity elements such as nitrogen to form blocky or angular TiN. Therefore, the formation of TiN has a nitrogen-fixing effect. However, excessive nitrogen content leads to excessively large blocky or angular TiN sizes, deteriorating the steel plate's properties and worsening the toughness of the weld heat-affected zone and the fatigue performance of the steel plate. Therefore, this invention limits N to ≤0.006% to avoid the formation of excessive blocky or angular TiN. Furthermore, the purpose of Ti microalloying in this invention is to enable Ti and C to form submicron or nanoscale fine spherical TiC particles for dispersed precipitation strengthening, refining the grains, strengthening the matrix, and achieving both grain refinement strengthening and precipitation strengthening. It can also strengthen ferrite, which is beneficial for obtaining excellent mechanical and fatigue properties, thereby improving the service life of steel in automotive structural components.

[0034] 3. Chromium can significantly improve the corrosion resistance of the coating and substrate of hot-dip galvanized steel sheets, inhibit the spread of coating corrosion, improve the wettability of zinc liquid, reduce coating defects, and enhance the strength and high-temperature stability of the substrate.

[0035] 4. Adding Mo to steel can delay the pearlite transformation, promote the formation of ferrite or bainite, and reduce banded structures. This improves the local formability of the steel sheet during stamping, preventing stamping cracks. It also significantly improves the tempering stability of steel, inhibiting the softening of martensite and bainite during tempering and ensuring that the strength does not decrease after galvanizing. Furthermore, Mo easily forms a dense oxide film on the substrate surface, reducing the substrate's corrosion tendency, resulting in a uniform galvanized layer, delaying the penetration of corrosive media, and improving resistance to atmospheric and salt water corrosion.

[0036] 5. Tungsten can refine grains, inhibit high-temperature oxidation, improve coating quality and substrate properties, and enhance the high-temperature strength of steel plates, reduce deformation during hot-dip galvanizing, improve coating adhesion and corrosion resistance, inhibit grain boundary oxidation, reduce incomplete coating defects, and improve the uniformity and surface quality of the galvanized layer.

[0037] 6. Copper can achieve precipitation strengthening by precipitating nanoscale precipitates, which can improve the strength, toughness and formability of steel plates. It can inhibit austenite grain growth, promote ferrite nucleation, refine grains and optimize the strength-ductility match; at the same time, it can lower the A3 temperature, expand the austenite region, help regulate the proportion of phase transformation structure, and improve the uniformity and stability of the structure.

[0038] 7. Zr forms high-melting-point, spherical ZrS, ZrO2, and ZrN with impurities such as S, O, and N, purifying grain boundaries, eliminating hot brittleness, improving hot workability, and reducing interference from impurities on the zinc-iron reaction. Zr segregates at grain boundaries and precipitates dispersed carbonitrides, strongly inhibiting the growth of austenite or ferrite grains, enhancing the strength and toughness of the matrix, and providing a good substrate for a uniform coating. Zr forms stable composite compounds with Si, reducing Si enrichment on the steel plate surface, inhibiting the over-catalysis of the Fe-Zn reaction by Si, avoiding problems such as excessively thick, dull, and poorly adhered coatings, and stabilizing the coating thickness and morphology.

[0039] 8. Cerium has a strong affinity for harmful elements such as oxygen and sulfur in steel, forming high-melting-point stable compounds such as Ce₂O₃ and CeS. These substances easily float and separate from molten steel, effectively reducing non-metallic inclusions and improving steel purity. For inclusions that cannot be completely removed, cerium can modify their morphology and distribution, transforming elongated sulfides that easily cause brittleness into spherical or dot-shaped cerium sulfides, mitigating the adverse effects of inclusions on the mechanical properties of steel, and significantly improving the toughness and fatigue strength of steel. Simultaneously, cerium can inhibit grain growth during heating and cooling, refining austenite or ferrite grains, thereby synergistically improving the strength and toughness of steel. Through this purification and modification effect, the plasticity and toughness of steel are improved, and the risk of cracking during hot and cold working is reduced; furthermore, it can optimize the weldability and oxidation resistance of steel, extending its service life in high-temperature environments.

[0040] 9. Germanium can improve the hot working properties of steel, reduce the tendency of steel to crack during rolling, and optimize the weldability of steel, reducing the brittleness of weld joints and improving the consistency of the mechanical properties of weld joints. It can also optimize the adhesion of the zinc layer, inhibiting excessive thickness of the Fe-Zn brittle alloy layer, reducing coating peeling, flaking, and zinc loss during stamping and bending. Furthermore, it can refine the coating structure, reduce porosity, making the zinc layer more uniform and dense, improving corrosion resistance, and improving the surface condition of the substrate, reducing oxidation defects, improving the wettability of the zinc bath, avoiding defects such as incomplete plating and pinholes, and ensuring the protective effect and smoothness of the coating.

[0041] 10. Praseodymium can remove harmful impurities such as O and S from steel, spheroidize brittle inclusions in the form of strips or angular shapes, reduce interface defects caused by inclusions during hot-dip galvanizing, improve zinc layer adhesion and stamping crack resistance, reduce the risk of coating peeling, refine substrate grains and improve the surface oxide layer state, inhibit abnormal thickening of Fe-Zn intermetallic compound layer during hot-dip galvanizing, promote coating microstructure refinement, reduce porosity and zinc flower size, and increase resistance to atmospheric corrosion and pitting corrosion.

[0042] 11. The microstructure of the steel of this invention consists of ferrite, bainite, martensite and retained austenite, thereby significantly improving the comprehensive mechanical properties of the steel plate during the forming process; 12. It does not require the conventional hot rolling + pickling + cold rolling + continuous annealing galvanizing + finishing process. Instead, it adopts the hot rolling + reduction descaling + continuous annealing galvanizing + finishing process, which reduces the pickling + cold rolling process, resulting in low production cost and no environmental pollution.

[0043] 13. The present invention has excellent mechanical properties, with yield strength ≥985MPa, tensile strength ≥1185MPa, longitudinal elongation A ≥25.5%, transverse cold bending 180° D=a qualified, and surface roughness Ra 0.90-1.60μm. Detailed Implementation

[0044] To make the objectives, technical solutions, and advantages of this invention clearer, the specific implementation methods of this invention will be further described below in conjunction with the embodiments. The following embodiments are used to specifically illustrate the content of this invention. These embodiments are only general descriptions of the content of this invention and do not limit the content of this invention.

[0045] The smelting process employs RH+LF, with strict control over H and O content: H ≤ 0.0002%, O ≤ 0.0015%. Calcium treatment is performed during the refining process. Electromagnetic stirring and light reduction techniques are used during continuous casting, with a billet casting speed ≤ 1.6 m / min and a light reduction of 2.0 mm-2.5 mm. Continuously cast slabs (220-270) mm thick × (1020-1700) mm wide are directly hot-charged into a walking beam furnace for heating at 1230-1245℃ for 150-200 min. The roughing process employs a 3+3 rolling mode (R1 is rolled in 3 passes, and R2 is rolled in 3 passes), for a total of 6 passes. The roughing mill exit temperature is 1090℃~1115℃, the intermediate slab thickness is 30.0~40.0mm, and the width is 1020~1700mm. The intermediate slab is insulated with a heat shield before entering the hot finishing mill to reduce temperature drop on the delay roller table and temperature differences at the beginning and end and across the slab width. The finishing mill is a 7-stand continuous rolling process. High-pressure water descaling is performed before finishing, with the finishing mill inlet temperature not exceeding 1090℃. The final rolling temperature is 880-945℃. After finishing, laminar flow cooling is used at a rate of approximately 18-28℃ / s. After cooling to 630-666℃, the slab is coiled and air-cooled to room temperature. The rolled thickness is 1.62-1.92mm. After cooling 1.62-1.92mm thick steel coils to room temperature, the iron oxide scale on the steel plate surface is descaled in a high-temperature continuous heating furnace. The descaling principle is as shown in Formulas 1-6. The main processes are uncoiling, heating reduction, surface treatment, cooling, leveling, and coiling. The heating reduction gas for the steel plate is a mixture of 60%-75% H2, 0%-10% CO, and 20%-40% N2, with a heating temperature of 680-1150℃. The surface treatment uses a heat-resistant steel wire brush to clean the reduced iron filings. The cooling gas is 100% N2, with a cooling temperature ≤50℃. The leveling elongation is 1.1-1.6%. Steel coils, after reduction descaling and with a thickness of 1.62~1.92mm, are produced using a continuous annealing galvanizing process. The process parameters are controlled as follows: belt speed 55~80m / min, soaking zone temperature 820~840℃, holding time 10~16min, slow cooling outlet temperature 705~720℃, rapid cooling rate greater than 45℃ / s, rapid cooling outlet temperature 450~460℃. The coils are then fed into a zinc bath at 455~465℃ for hot-dip galvanizing, with a galvanizing time of 1~6s. After galvanizing, the coils enter a finishing machine, with a finishing elongation of 1.1-1.6%. The finished product thickness is 1.62-1.92mm.

[0046] The specific components, hot rolling process, reduction descaling process, continuous annealing galvanizing process, steel plate properties, and volume percentage of the six embodiments of the present invention are shown in Tables 1-6.

[0047] Table 1 Chemical composition (wt, %) of embodiments of the present invention

[0048] Table 1 Chemical composition (wt, %) of embodiments of the present invention (continued)

[0049] Table 2 Hot rolling process of embodiments of the present invention

[0050] Table 3. Reduction and descaling process of the present invention embodiments

[0051] Table 4. Continuous annealing and galvanizing process of the present invention.

[0052] Table 5 Mechanical performance parameters of embodiments of the present invention

[0053] Table 6. Percentage of tissue volume in embodiments of the present invention .

Claims

1. A type of ultra-high strength, acid-free, hot-dip galvanized automotive steel, characterized in that, The chemical composition of the steel, by weight percentage, is as follows: C: 0.102%–0.162%, Si: 0.15%–0.30%, Mn: 2.00%–2.22%, Al: 0.035%–0.056%, Nb: 0.053%–0.072%, Ti: 0.130%–0.145%, Cr: 0.50%–0.60%, Mo: 0.30%–0.45%, W: 0.015%–0.042%, Cu: 0.16%–0. 0.22%, Zr: 0.015%~0.025%, Ce: 0.0035%~0.0082%, Ge: 0.0055%~0.012%, Pr: 0.016%~0.022%, and C / Nb: 1.50~2.50, C / Ti: 0.70~1.14, Mo+Cr: 0.85%~1.00%, with P≤0.012%, S≤0.005%, N≤0.006%, and the balance being Fe and unavoidable impurities.

2. The ultra-high strength, acid-free, hot-dip galvanized automotive steel according to claim 1, characterized in that, The finished steel plate has a ferrite volume percentage of 2%–10%, a bainite volume percentage of 10%–20%, a martensite volume percentage of 60%–70%, and a retained austenite volume percentage of 5%–10%.

3. The ultra-high strength, acid-free, hot-dip galvanized automotive steel according to claim 1, characterized in that, The finished steel plate has a yield strength ≥985MPa, tensile strength ≥1185MPa, longitudinal elongation A ≥25.5%, transverse cold bending 180° D=a qualified, surface roughness Ra is 0.90~1.60μm, and the thickness of the finished steel plate is 1.62~1.92mm.

4. A method for producing ultra-high strength, acid-free, hot-dip galvanized automotive steel as described in any one of claims 1-3, characterized in that, The processes include smelting, hot rolling, reduction descaling, continuous annealing galvanizing, and finishing. Specific methods include: 1) Reduction Descaling: After the hot-rolled steel coil is cooled to room temperature, the iron oxide scale on the surface of the steel plate is reduced and descaled in a continuous heating furnace. The process includes uncoiling, heating reduction, surface treatment, cooling, leveling, and coiling. The heating reduction atmosphere of the steel plate is a mixture of 60% to 75% H2, 0% to 10% CO, and 20% to 40% N2 by volume. The heating temperature is 680 to 1150℃. The surface treatment uses a heat-resistant steel wire roller brush to clean the reduced iron filings on the surface of the steel plate. The cooling gas is 100% N2, the cooling temperature is ≤50℃, and the leveling elongation is 1.1% to 1.6%. 2) Continuous annealing and galvanizing: The steel coils after reduction and descaling are produced by continuous annealing and galvanizing process. The process parameters are controlled as follows: belt speed 55~80m / min, soaking zone temperature 820~840℃, holding temperature 10~16min, slow cooling outlet temperature 705~720℃, rapid cooling rate greater than 45℃ / s, rapid cooling outlet temperature 450~460℃, and then sent into a zinc pot at 455~465℃ to complete hot galvanizing, with a galvanizing time of 1~6s.

5. The production method of ultra-high strength acid-free hot-dip galvanized automotive steel according to claim 4, characterized in that, After reduction and descaling, the volume percentage of ferrite in the steel plate is 30%–38%, the volume percentage of lamellar pearlite is 10%–64%, and the volume percentage of spheroidized pearlite is 0%–60%.

6. The production method of ultra-high strength acid-free hot-dip galvanized automotive steel according to claim 4, characterized in that, The thickness of the hot-rolled steel coil is 1.62 to 1.92 mm.

7. The production method of ultra-high strength acid-free hot-dip galvanized automotive steel according to claim 4, characterized in that, The smelting process includes: using the RH+LF process, incorporating electromagnetic stirring and light reduction technology during continuous casting, with a billet casting speed ≤1.6m / min and a light reduction amount of 2.0mm~2.5mm.

8. The production method of ultra-high strength acid-free hot-dip galvanized automotive steel according to claim 4, characterized in that, The hot rolling process includes: hot-charging the continuously cast slab into a heating furnace for heating at a temperature of 1230–1245°C for a holding time of 150–200 min; the roughing mill exit temperature is 1090°C–1115°C; the intermediate slab is kept warm by an insulation cover before entering the hot rolling finishing mill; the finishing mill inlet temperature is not higher than 1090°C; the final rolling temperature is 880–945°C; after final rolling, laminar flow cooling is adopted at a rate of 18–28°C / s; after cooling to 630–666°C, the slab is coiled and air-cooled to room temperature.

9. The production method of ultra-high strength acid-free hot-dip galvanized automotive steel according to claim 8, characterized in that, The continuous casting slab has a thickness of 220-270 mm and a width of 1020-1700 mm, while the intermediate slab has a thickness of 30.0-40.0 mm and a width of 1020-1700 mm.

10. The production method of ultra-high strength acid-free hot-dip galvanized automotive steel according to claim 4, characterized in that, The smoothing elongation is between 1.1% and 1.6%.

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