A 1565mpa grade hot-dip galvannealed automotive steel without pickling and a production method thereof

Abstract

The present application belongs to the field of metal materials, and particularly relates to a 1565MPa-grade pickling-free hot galvanizing automobile steel and a production method thereof. The steel plate of the present application is mainly suitable for manufacturing automobile structural parts. The chemical composition of the steel is composed of C, Si, Mn, Al, V, Ti, Cr, Mo, W, Cu, Ni, Zr, Ce, Ge and Pr, and C / V is 0.85-1.40, C / Ti is 1.90-2.60, Mo+Cr is 1.15%-1.30%, and Ce+Pr is 0.019%-0.027%. The yield strength of the steel plate of the present application is greater than or equal to 1298MPa, the tensile strength is greater than or equal to 1565MPa, the longitudinal elongation A is greater than or equal to 19%, the cold bending 180 D=a is qualified, the surface quality is good, there is no color difference defect, and the surface roughness Ra is 1.00-1.30μm. The pickling + cold rolling process is reduced, the production cost is low, and the environment is not polluted.

Description

Technical Field

[0001] This invention belongs to the field of metallic materials, and specifically relates to a 1565MPa grade 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 CN113388773A discloses a 1.5GPa grade high-formability, hydrogen embrittlement-resistant ultra-high-strength automotive steel and its preparation method. It utilizes a common C-Mn composition system with added amounts of Mo, Cu, Nb, and V to produce a hot-rolled, pickled, cold-rolled, continuously annealed, and finished steel sheet. The sheet exhibits a yield strength of 1100–1350 MPa, a tensile strength of 1470–1650 MPa, and an A80 elongation after fracture ≥7.0%, which does not meet the high elongation requirements for 1565 MPa grade automotive parts. Furthermore, the process involves hot rolling, pickling, cold rolling, continuous annealing, and finishing, resulting in a large process flow, high production costs, and, in particular, the need for pickling. The pickling process generates significant acid mist pollution, leading to environmental pollution and high waste acid treatment costs.

[0004] Chinese patent application CN 113388779A discloses a 1.5GPa grade ultra-high strength, high plasticity, and high-perforation DH steel plate and its preparation method. Utilizing a common C-Mn composition system with the addition of certain amounts of Nb, Ti, Cr, and V, the resulting hot-rolled, pickled, cold-rolled, continuously annealed, and finished steel plate has a strength of 1000–1350 MPa, a tensile strength of 1470–1650 MPa, and an A80 elongation after fracture ≥9.0%. This does not meet the high elongation processing requirements for 1565 MPa grade automotive parts. Furthermore, the manufacturing process involves hot rolling, pickling, cold rolling, continuous annealing, and finishing, which is complex, costly, and particularly problematic due to the need for pickling, which generates significant acid mist pollution, leading to environmental pollution and high waste acid treatment costs. Summary of the Invention

[0005] To address the development needs of the automotive steel industry, this invention provides a 1565MPa grade pickling-free hot-dip galvanized automotive steel and its production method. The steel exhibits a yield strength ≥1298MPa, tensile strength ≥1565MPa, longitudinal elongation A ≥19%, and meets the acceptable transverse cold bending standard (D=a at 180°). It also boasts good surface quality, free of color defects, and a surface roughness Ra of 1.00-1.30μm. 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 1565MPa grade, acid-free hot-dip galvanized automotive steel, with the following chemical composition by weight percentage: C: 0.29%–0.36%, Si: 0.10%–0.21%, Mn: 2.35%–2.65%, Al: 0.038%–0.055%, V: 0.245%–0.356%, Ti: 0.135%–0.156%, Cr: 0.56%–0.68%, Mo: 0.56%–0.67%, W: 0.015%–0.049%, Cu: 0.08%–0.15%, Ni: 0. The composition is 0.011%–0.056%, Zr: 0.012%–0.021%, Ce: 0.0033%–0.0062%, Ge: 0.0072%–0.013%, Pr: 0.016%–0.022%, with C / V: 0.85–1.40, C / Ti: 1.90–2.60, Mo+Cr: 1.15%–1.30%, Ce+Pr: 0.019%–0.027%, and P≤0.012%, S≤0.005%, N≤0.006%, with the balance being Fe and unavoidable impurities.

[0007] The finished steel plate has a microstructure consisting of 3%–6% bainite, 85%–89% martensite, 2%–4% tempered martensite, and 2%–6% retained austenite. The finished steel plate has a yield strength ≥1298 MPa, tensile strength ≥1565 MPa, longitudinal elongation A ≥19%, a transverse cold bending radius of 180° D=a (qualified), a surface roughness Ra of 1.00–1.30 μm, and a finished plate thickness of 1.70–2.35 mm.

[0008] The main functions of the chemical components in this invention are: C: Carbon is a key strengthening element in steel, achieving solid solution strengthening by inducing lattice distortion through interstitial solid solution. In this invention, the core role of carbon is to regulate the ratio of bainite to martensite in the final microstructure and to promote the diffusion of carbon from martensite to retained austenite, thereby enhancing austenite stability, increasing the volume fraction of retained austenite, and thus improving the formability of the steel plate. Too low a carbon content will not meet the mechanical properties required by this invention; too high a carbon content will increase the brittleness of the steel plate, increasing the risk of delayed fracture and hot-rolling edge cracking, while also deteriorating weldability, plasticity, and toughness. Furthermore, excessive carbon will make the retained austenite overly stable, making it difficult for the TRIP effect to occur effectively during subsequent deformation, weakening work hardening ability, and reducing the strength of the steel plate. This invention controls the carbon content within a low-carbon range, which can reduce the risk of delayed fracture and hot-rolling edge cracking, improve weldability, and ensure that the retained austenite fully exerts the TRIP effect during deformation, balancing strength and formability. Therefore, the optimal range of carbon in this invention is 0.29-0.36%.

[0009] Si: In this invention, silicon is a key element. As a solid solution strengthening element, silicon can significantly improve the strength of steel plates. Simultaneously, it can effectively improve the hardenability of steel plates. During continuous annealing, silicon can inhibit cementite precipitation and promote the enrichment of carbon from martensite to untransformed austenite, thereby enhancing the stability of austenite. Furthermore, an appropriate amount of silicon can purify the steel, reduce the formation of inclusions, and prevent them from damaging the mechanical properties of the steel plate. However, the silicon content needs to be strictly controlled; too low a content will not effectively suppress inclusions, while too high a content will deteriorate the surface quality of hot-rolled steel, leading to increased iron oxide scale and reduced weldability. Therefore, the silicon content in this invention is 0.10-0.21%.

[0010] Mn: Manganese in steel achieves solid solution strengthening by causing lattice distortion through substitutional solid solution. Simultaneously, as an austenite stabilizing element, it can expand the austenite phase region, reduce the critical quenching rate of steel, and delay the transformation of austenite to pearlite. However, if the manganese content is too low, the stability of supercooled austenite is insufficient, leading to a decrease in the plasticity and toughness of the steel plate. If the manganese content exceeds a suitable range, it will bring a series of problems: on the one hand, it easily induces carbon and manganese segregation, disrupting the uniformity of the microstructure during hot rolling and easily forming severe banded defects; on the other hand, it will excessively increase the hardenability of the steel, thus inhibiting the formation of bainite; furthermore, excessively high manganese content will significantly reduce the weldability of the steel plate. Therefore, considering all factors, this invention selects a manganese content of 2.35-2.65%.

[0011] 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%.

[0012] 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%.

[0013] In traditional steelmaking processes, aluminum (Al) is primarily used as a deoxidizer. It can also combine with nitrogen (N) in steel to form aluminum nitride (AlN), refining the grain size and inhibiting the decomposition of residual austenite. Furthermore, aluminum works synergistically with silicon (Si) to suppress cementite precipitation, increasing the austenitizing temperature of the steel. This helps optimize the selection range of process parameters and promotes bainite transformation. However, excessive aluminum content can cause clogging of continuous casting nozzles, thereby reducing production efficiency and increasing production costs. Therefore, in this invention, the Al content is limited to 0.038-0.055%.

[0014] Vanadium (V) added to steel primarily achieves grain refinement and precipitation strengthening by forming V(C,N) nano-precipitates, inhibiting austenite grain growth, and improving strength, plasticity, and toughness. V(C,N) pins grain boundaries, reducing grain coarsening during high-temperature heating, resulting in a more uniform surface, which is beneficial for zinc immersion, reduces plating defects, inhibits silicon or manganese enrichment, reduces brittle interfacial phases, and improves coating adhesion and formability. This leads to a more uniform substrate structure, a denser zinc coating, and reduced surface blackening. Therefore, the optimal V content range in this invention is between 0.245% and 0.356%, satisfying a C / V ratio of 0.85-1.40.

[0015] 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-scale 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. This is beneficial for 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 0.135-0.156%, and it satisfies C / Ti: 1.90-2.60.

[0016] 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 0.56-0.68%.

[0017] Mo: Adding Mo to steel can delay the pearlite transformation, promote bainite formation, 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 0.56% and 0.67%, and satisfies the Mo+Cr ratio of 1.15% to 1.30%.

[0018] 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% and 0.049%.

[0019] 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, 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 segregate 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 0.08-0.15%.

[0020] Ni (Ni): As a solid solution strengthening element, nickel can improve the hardenability and fatigue performance of steel, effectively prevent temper brittleness, and enhance the corrosion resistance of steel. It has no adverse effect on the hardening and toughness of the heat-affected zone in steel welding, and as an austenite stabilizing element, it helps promote the retention of retained austenite. In hot-dip galvanizing, nickel can significantly inhibit the interdiffusion at the iron-zinc interface, prevent abnormal growth of the alloy layer, thereby improving the adhesion and uniformity of the coating, optimizing the corrosion resistance of the coating, and making the free zinc layer more continuous and brighter. However, Ni is a valuable element, and its content should not be too high. Therefore, the Ni content is limited to 0.011%–0.056%.

[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 austenite grain growth, 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 excessive catalysis of the Fe-Zn reaction by Si, avoiding problems such as excessively thick coatings, dullness, and poor adhesion, and stabilizing coating thickness and morphology. Therefore, the optimal range of Zr content in this invention is between 0.012% and 0.021%.

[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 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. Therefore, this invention limits the Ce content to 0.0033-0.0062%.

[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 0.0072% and 0.013%.

[0024] Praseodymium (Pr) can remove harmful impurities such as oxygen (O) and sulfur (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 condition, inhibit abnormal thickening of the 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, this invention limits the Pr content to 0.016-0.022%, and satisfies Ce+Pr: 0.019-0.027%.

[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 and V(C,N) formed with Ti and V 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 1565MPa grade 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 ≤1.2m / min and the light reduction amount is 1.5mm~2.0mm to reduce the center segregation of the continuous casting billet, which is beneficial to the reduction of banded structure in the subsequent hot-rolled steel plate.

[0027] (2) Hot rolling process: The continuous casting slab with a thickness of 200-230 mm and a width of 1020-1380 mm is directly hot-loaded into the walking beam furnace for heating at a temperature of 1225-1240℃ and a holding time of 180-220 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℃~1110℃, the intermediate slab thickness is 34.0~49.0mm, and the width is 1020~1380mm. 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 895~948℃. After final rolling, laminar flow cooling is used at a rate of approximately 15~25℃ / s. After cooling to 645~660℃, the slab is coiled and air-cooled to room temperature. The rolled thickness is 1.70~2.35mm.

[0028] (3) Reduction Descaling: After the hot-rolled steel coil with a thickness of 1.70-2.35mm 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 55%-75% H2, 0%-15% CO and 20%-35% N2 by volume. The heating temperature is 700-1100℃. 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 ≤40℃. The leveling elongation is 1.0-1.5%. After reduction descaling, the volume percentage of ferrite is 20%-28%, the volume percentage of lamellar pearlite is 11%-75%, and the volume percentage of spheroidized pearlite is 0%-69%.

[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: Continuous annealing is performed on steel coils with a thickness of 1.70–2.35 mm after reduction and descaling treatment. The strip speed is controlled at 82–93 m / min, the soaking temperature is set at 850–880℃, and the soaking time is 5–10 min. The slow cooling outlet temperature is controlled at 500–550℃, the rapid cooling rate is ≥55℃ / s, and the rapid cooling outlet temperature is 275–325℃. After rapid cooling, the temperature is raised to 425–475℃ at a rate of 20–28℃ / s for over-aging treatment, and the over-aging time is 5–9 min. Then, the coils are sent to a zinc pot at 450–460℃ for hot-dip galvanizing, and the galvanizing time is 1–6 s. The soaking temperature is controlled at 850–880℃, mainly to ensure complete austenitization of the microstructure, providing conditions for obtaining no less than 85% martensite in the subsequent rapid cooling stage, while avoiding excessively high soaking temperatures that could cause coarsening of austenite grains, thereby ensuring the final strength. The soaking time is set to 5–10 min to allow the steel plate to fully recrystallize during heating and holding, while preventing excessive holding time from causing grain growth. If the soaking time is too short, the steel plate cannot fully complete continuous annealing and recrystallization, leading to a decrease in elongation and hindering the formation of a complete austenitic structure. The slow cooling outlet temperature is controlled at 500–550℃ to promote rapid bainite precipitation, ensuring bainite content while suppressing grain growth, thus achieving bainite grain refinement. The rapid cooling outlet temperature is controlled at 275–325℃ to promote rapid martensite precipitation, similarly ensuring martensite content while suppressing grain growth, thus achieving martensite grain refinement. The over-aging temperature is set to 425–475℃ for 5–9 min to allow carbon in the martensite to diffuse into the retained austenite, improving the stability of the retained austenite and increasing its volume fraction, thereby enhancing the plasticity of the steel plate. If the aging temperature is too low or the aging time is too short, the diffusion rate of carbon into the retained austenite will slow down, which will reduce the stability and content of the retained austenite, thereby affecting the plasticity of the steel plate.

[0031] (5) Finishing: After galvanizing, the product enters the finishing machine, and the finishing elongation is 1.0% to 1.5%.

[0032] Compared with the prior art, the beneficial effects of the present invention are: 1. The addition of vanadium (V) to steel mainly achieves grain refinement and precipitation strengthening by forming V(C,N) nano-precipitates, inhibiting austenite grain growth, and improving strength, plasticity, and toughness. V(C,N) pins grain boundaries, reducing grain coarsening during high-temperature heating, resulting in a more uniform surface, which is beneficial for zinc liquid wetting, reducing plating defects, inhibiting silicon or manganese enrichment, reducing brittle phases at the interface, improving coating adhesion and formability, making the substrate structure more uniform, the zinc coating denser, and reducing surface blackening.

[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 effects. This 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 bainite formation, and reduce banded structures. This 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, 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, 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. Nickel, as a solid solution strengthening element, can improve the hardenability and fatigue performance of steel, effectively prevent temper brittleness, and enhance the corrosion resistance of steel. It has no adverse effect on the hardening and toughness of the heat-affected zone in steel welding, and as an austenite stabilizing element, it helps promote the retention of retained austenite. In the hot-dip galvanizing process, nickel can also significantly inhibit the interdiffusion at the iron-zinc interface, prevent abnormal growth of the alloy layer, thereby improving the adhesion and uniformity of the coating, optimizing the corrosion resistance of the coating, and making the free zinc layer more continuous and brighter.

[0039] 8. 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 austenite grain growth, 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 excessive catalysis of the Fe-Zn reaction by Si, avoiding problems such as excessively thick coatings, dullness, and poor adhesion, and stabilizing coating thickness and morphology.

[0040] 9. 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 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.

[0041] 10. 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, 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.

[0042] 11. 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.

[0043] 12. The microstructure of the steel of this invention consists of bainite, martensite, tempered martensite, and retained austenite, thereby significantly improving the comprehensive mechanical properties of the steel plate during the forming process.

[0044] 13. 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.

[0045] 14. The present invention has excellent mechanical properties, with a yield strength ≥1298MPa, tensile strength ≥1565MPa, longitudinal elongation A ≥19%, transverse cold bending 180° D=a qualified, and surface roughness Ra 1.00-1.30μm. Detailed Implementation

[0046] 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.

[0047] 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 ≤ 1.2 m / min and a light reduction of 1.5 mm-2.0 mm. Continuously cast slabs, 200-230 mm thick and 1020-1380 mm wide, are directly hot-charged into a walking beam furnace for heating at 1225-1240℃ for 180-220 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℃~1110℃, the intermediate slab thickness is 34.0~49.0mm, and the width is 1020~1380mm. 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 in the width direction. 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 895-948℃. After finishing, laminar flow cooling is used at a rate of approximately 15-25℃ / s. After cooling to 645-660℃, the slab is coiled and air-cooled to room temperature. The rolled thickness is 1.70-2.35mm. After cooling 1.70-2.35mm 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 55%-75% H2, 0%-15% CO, and 20%-35% N2, with a heating temperature of 700-1100℃. 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 ≤40℃. The leveling elongation is 1.0-1.5%. Steel coils, after reduction descaling and with a thickness of 1.70~2.35mm, are produced using a continuous annealing galvanizing process. The process parameters are controlled as follows: belt speed is controlled at 82~93m / min; the soaking zone temperature is set at 850~880℃ for 5~10min; the slow cooling outlet temperature is controlled at 500~550℃; the rapid cooling rate is greater than 55℃ / s for 275~325℃; after rapid cooling, the temperature is increased to 425~475℃ at a rate of 20~28℃ / s for over-aging treatment for 5~9min; subsequently, the coils are sent to a zinc pot at 450~460℃ for hot-dip galvanizing for 1~6s; after galvanizing, the coils enter a finishing machine with a finishing elongation of 1.0-1.5%. The finished product thickness is 1.70-2.35mm.

[0048] 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.

[0049] Table 1 Chemical composition (wt, %) of embodiments of the present invention

[0050] Table 1 Chemical composition (wt, %) of embodiments of the present invention (continued)

[0051] Table 2 Hot rolling process of embodiments of the present invention

[0052] Table 3. Reduction and descaling process of the present invention embodiments

[0053] Table 4. Continuous annealing and galvanizing process of the present invention.

[0054] Table 5 Mechanical performance parameters of embodiments of the present invention

[0055] Table 6. Percentage of tissue volume in embodiments of the present invention

Claims

1. A 1565MPa grade acid-free hot-dip galvanized automotive steel, characterized in that, The chemical composition of the steel, by weight percentage, is as follows: C: 0.29%–0.36%, Si: 0.10%–0.21%, Mn: 2.35%–2.65%, Al: 0.038%–0.055%, V: 0.245%–0.356%, Ti: 0.135%–0.156%, Cr: 0.56%–0.68%, Mo: 0.56%–0.67%, W: 0.015%–0.049%, Cu: 0.08%–0.15%, Ni: 0.011%–0.056%. %, Zr: 0.012%~0.021%, Ce: 0.0033%~0.0062%, Ge: 0.0072%~0.013%, Pr: 0.016%~0.022%, and C / V: 0.85~1.40, C / Ti: 1.90~2.60, Mo+Cr: 1.15%~1.30%, Ce+Pr: 0.019%~0.027%, with P≤0.012%, S≤0.005%, N≤0.006%, and the balance being Fe and unavoidable impurities.

2. The 1565MPa grade acid-free hot-dip galvanized automotive steel according to claim 1, characterized in that, The finished steel plate has a bainite volume percentage of 3%–6%, a martensite volume percentage of 85%–89%, a tempered martensite volume percentage of 2%–4%, and a retained austenite volume percentage of 2%–6%.

3. The 1565MPa grade acid-free hot-dip galvanized automotive steel according to claim 1, characterized in that, The finished steel plate has a yield strength ≥1298MPa, tensile strength ≥1565MPa, longitudinal elongation A ≥19%, transverse cold bending 180° D=a qualified, surface roughness Ra is 1.00~1.30μm, and finished steel plate thickness is 1.70~2.35mm.

4. A method for producing 1565MPa grade 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 high-temperature 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 55%–75% H2, 0%–15% CO, and 20%–35% N2 by volume, and the heating temperature is 700–1100℃. 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 ≤40℃, and the leveling elongation is 1.0–1.5%. 2) Continuous annealing and galvanizing: The steel coils that have undergone reduction and descaling are continuously annealed, with the belt speed controlled at 82-93 m / min, the soaking temperature set at 850-880℃, and the soaking time at 5-10 min; the slow cooling outlet temperature is controlled at 500-550℃, the rapid cooling rate is ≥55℃ / s, and the rapid cooling outlet temperature is 275-325℃; after rapid cooling, the temperature is raised to 425-475℃ at a rate of 20-28℃ / s for over-aging treatment, with an over-aging time of 5-9 min, and then sent to a zinc pot at 450-460℃ for hot-dip galvanizing, with a galvanizing time of 1-6 s.

5. The production method of 1565MPa grade 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 20%–28%, the volume percentage of lamellar pearlite is 11%–75%, and the volume percentage of spheroidized pearlite is 0%–69%.

6. The production method of 1565MPa grade acid-free hot-dip galvanized automotive steel according to claim 4, characterized in that, The thickness of the hot-rolled steel coil is 1.70 to 2.35 mm.

7. The production method of 1565MPa grade 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 ≤1.2m / min and a light reduction amount of 1.5mm~2.0mm.

8. The production method of 1565MPa grade 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 1225–1240°C for a holding time of 180–220 min; the roughing mill exit temperature is 1090°C–1110°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 895–948°C; after final rolling, laminar flow cooling is adopted at a rate of 15–25°C / s; after cooling to 645–660°C, the slab is coiled and air-cooled to room temperature.

9. The production method of 1565MPa grade acid-free hot-dip galvanized automotive steel according to claim 8, characterized in that, The continuous casting slab has a thickness of 200-230 mm and a width of 1020-1380 mm, while the intermediate slab has a thickness of 34.0-49.0 mm and a width of 1020-1380 mm.

10. The production method of 1565MPa grade acid-free hot-dip galvanized automotive steel according to claim 4, characterized in that, The smoothing elongation is between 1.0% and 1.5%.

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