An ultra-high strength high-elongation hot-rolled pickled steel sheet for automobiles and a method for producing the same

Abstract

The present application belongs to the field of metal material, and particularly relates to a kind of ultra-high strength high elongation hot-rolled pickled steel plate for automobile and a production method thereof.The steel plate of the present application is mainly suitable for manufacturing automobile chassis structural parts.The chemical composition of the steel is designed as follows: C, Si, Mn, Al, V, Ti, Cr, Ni, Cu, Ta, Y, and Al / Si: 0.8-1.5, C / V: 0.45-0.7, C / Ti: 2.4-3.3, and P≤0.015%, S≤0.005%, N≤0.006% are limited, and the balance is Fe and inevitable impurities.The yield strength of the steel plate of the present application is≥735MPa, the tensile strength is≥1155MPa, A is≥28%, the hole expansion ratio is≥46%, the longitudinal cold bending 180°D=a is qualified, the surface quality is good, there are no iron oxide scale stripes and color difference defects, Ra is 0.78-1.40μm, and the steel plate has good corrosion resistance.

Description

Technical Field

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

[0002] In recent years, with the development of the automotive industry, automakers have increasingly demanded more personalized steel materials. Hot-rolled pickled steel sheet is an intermediate product made from high-quality hot-rolled steel sheet. After removing the iron oxide scale through a pickling unit, its surface quality and usage requirements fall between those of hot-rolled and cold-rolled steel sheets. It is an ideal substitute for some hot-rolled and cold-rolled steel sheets. Currently, given the challenging steel market conditions, hot-rolled pickled steel sheet offers better surface quality than hot-rolled steel sheet and, while maintaining the required surface quality compared to cold-rolled steel sheet, effectively reduces procurement costs for users, making it one of the most profitable products for steel companies.

[0003] Chinese patent application CN104878178B discloses a method for producing strapping with a tensile strength ≥1000MPa. The method involves cold rolling a pickled 30Mn2 hot-rolled coil; uncoiling, slitting, and deburring followed by high-temperature austenitization; quenching in a mixed solution of Triathon X-100, Tween 20, and water; tempering after conventional water washing; natural air cooling to room temperature; and finally, coiling. The product of this invention has a thickness of 0.8~1.0mm, a tensile strength of 1030~1070MPa, an elongation of 10~12%, and withstands at least 4 repeated bending cycles (R=2.5mm). It also exhibits a uniform, dense, and strongly adherent bluing layer with a thickness of 4~6µm on its surface. This product meets the requirements for bundling and packaging large, heavy hot-rolled steel coils. However, it is not suitable for automotive chassis structural components.

[0004] Chinese patent application CN108796375A discloses a hot-dip galvanized high-strength steel with a tensile strength of 1000MPa and its reduced-volume production method. The product's microstructure mainly consists of 85-95% ferrite, 0-10% bainite, and 0-10% martensite, with 0.2-1% volume percentage of precipitates ≤10nm in the steel. The production method involves steelmaking, refining, continuous casting, hot rolling, pickling, annealing, galvanizing, and finishing. This eliminates the cold rolling step in the traditional production process, significantly shortening the production flow, greatly reducing energy consumption and CO2 emissions, and significantly improving production efficiency. However, this steel plate product is not suitable for the application requirements of automotive chassis structural components. Summary of the Invention

[0005] To address the development needs of the automotive steel industry, this invention provides an ultra-high strength, high elongation hot-rolled pickled steel sheet for automobiles and its production method. The steel sheet has a yield strength ≥735MPa, tensile strength ≥1155MPa, longitudinal elongation A ≥28%, hole expansion rate ≥46%, and meets the acceptable longitudinal cold bending standard (D=a) at 180°. It also exhibits a surface roughness Ra of 0.78~1.40μm and good corrosion resistance. Using an alkaline solution of 3.7g / L NaOH + 10.5g / L KOH + 2g / L Ca(OH)2, the corrosion rate of the steel sheet is ≤0.41g / (m²) over a 168h test period. 2 ·h).

[0006] To achieve the above objectives, the present invention employs the following technical solution:

[0007] A type of ultra-high strength, high elongation hot-rolled pickled steel sheet for automotive applications, with the following chemical composition by weight percentage: C: 0.240%–0.270%, Si: 0.40%–0.62%, Mn: 1.85%–2.60%, Al: 0.45%–0.66%, V: 0.35%–0.55%, Ti: 0.075%–0.110%, Cr: 0.40%–0.65%, Ni: 0. 150%~0.190%, Cu: 0.160%~0.200%, Ta: 0.10%~0.32%, Y: 0.003%~0.010%, and Al / Si: 0.8~1.5, C / V: 0.45~0.7, C / Ti: 2.4~3.3, and limited P≤0.015%, S≤0.005%, N≤0.006%, with the balance being Fe and unavoidable impurities.

[0008] The microstructure of the finished steel plate of this invention consists of 0-8% ferrite, 16-19% bainite, 73-82% martensite, and 1-4% retained austenite. The steel plate has a yield strength ≥735MPa, tensile strength ≥1155MPa, longitudinal elongation A ≥28%, hole expansion rate ≥46%, and passes the longitudinal cold bending test (D=a) at 180°. The finished steel plate has a thickness of 1.50-6.00mm and a surface roughness Ra of 0.78-1.40μm. Using an alkaline solution of 3.7g / L NaOH + 10.5g / L KOH + 2g / L Ca(OH)₂, the corrosion rate of the steel plate is ≤0.41g / (m²) over a 168h test period. 2 ·h).

[0009] The main function of the composition of the ultra-high strength, high elongation hot-rolled pickled steel sheet for automobiles in this invention is as follows:

[0010] C: Carbon is a common strengthening element in steel. Interstitial solid solution carbon atoms cause certain lattice distortion in the matrix, playing a role in solid solution strengthening. In this invention, carbon ensures the content of bainite and martensite. Simultaneously, after coiling, it enters a heated insulation pit, causing carbon from the martensite to diffuse into the retained austenite, improving austenite stability and increasing the percentage of retained austenite, thereby improving the plasticity of the steel plate and consequently improving its formability and hole-expanding properties. If the carbon content is too low, the mechanical properties of the steel plate in this invention cannot be obtained; if the content is too high, the steel plate will become brittle, posing a risk of delayed fracture and hot-rolling edge cracking, and also negatively impacting the weldability, plasticity, and toughness of the steel plate. In this invention, the overall carbon content is required to be within a low range, which helps reduce the risk of delayed fracture and hot-rolling edge cracking, and is also beneficial to the weldability of the steel plate. Therefore, the optimal range of carbon in this invention is 0.240% to 0.270%.

[0011] Silicon (Si): Silicon is one of the key elements in this invention. Sufficient silicon addition ensures the strength of the ferrite matrix. Simultaneously, silicon inhibits cementite precipitation during coiling, ensuring carbon enrichment from martensite to untransformed austenite and improving austenite stability. Sufficient silicon content also reduces inclusions in the steel, inhibiting their formation and preventing a decrease in the mechanical properties and hole-expanding performance of the steel plate due to inclusion formation. However, too low a silicon content fails to guarantee the strength of the ferrite matrix and inhibit inclusion formation, while too high a content affects the surface quality of hot-rolled steel, resulting in a large amount of iron oxide scale. Therefore, the silicon content in this invention is 0.40%–0.62%.

[0012] Mn: Manganese strengthens the solid solution in steel by inducing lattice distortion through substitution solid solution. It is also an austenite stabilizing element in steel, expanding the austenite region, reducing the critical quenching rate of steel, and delaying the transformation of austenite to pearlite. However, if the manganese content is too low, the supercooled austenite is unstable, reducing the plasticity, toughness, and hole-expanding performance of the steel plate. Furthermore, the added manganese content should not exceed the scope of this invention, mainly considering the problem of C or Mn segregation caused by excessive manganese content, which deteriorates the uniformity of the steel plate structure during hot rolling and easily leads to severe banded structural defects. In addition, excessive manganese in the steel involved in this invention increases hardenability, inhibits bainite formation, and is also detrimental to hole-expanding performance. Moreover, excessive manganese content will lead to poor weldability of the steel plate. Therefore, considering all factors, this invention selects a manganese content of 1.85% to 2.60%.

[0013] P: Phosphorus is an impurity element in steel. It tends to agglomerate at grain boundaries. When the phosphorus content in steel is high, Fe2P particles are easily formed, which reduces the plasticity and toughness of the steel. Therefore, the lower its content, the better. In order to obtain a higher elongation, its upper limit is set at 0.015%.

[0014] S: Sulfur is an impurity element in steel. It easily combines with Mn to form MnS inclusions, which become the starting point of cracks and deteriorate the processing performance, seriously affecting the plasticity and formability of steel plates. Therefore, the lower the content, the better. The upper limit is set at 0.005%.

[0015] Al: In traditional steelmaking processes, Al acts as a deoxidizer, deoxidizing and nitrogen-fixing agent, effectively improving the oxidation resistance of steel plates. Al can also combine with nitrogen in steel to form AlN, refining grain size. Simultaneously, Al, along with Si, inhibits cementite precipitation, increases the austenitizing temperature, facilitates better selection of the process window, and accelerates bainite transformation. Excessive Al content can cause nozzle blockage during continuous casting, affecting production efficiency and increasing production costs. Therefore, in this invention, the Al content is limited to 0.45%–0.66%, and the Al / Si ratio is set at 0.8–1.5. The main purpose is to avoid deterioration in the weldability of subsequent products: excessive aluminum increases the hardenability of steel, easily producing hard and brittle structures during welding, leading to weld cracks; excessive silicon reduces the weldability of steel (easily producing porosity) and causes "silicon spots" on the steel plate surface, affecting surface quality. Therefore, limiting the Al / Si ratio ensures weld stability. This invention also aims to achieve an optimal balance in deoxidation effect, grain refinement, strength, plasticity, and processing performance to meet application requirements.

[0016] Vanadium (V) has significant precipitation strengthening and grain refinement effects. Its effect is mainly achieved through the formation of precipitates with carbon and nitrogen, especially the VN precipitation formed with nitrogen, which greatly improves the strength of the steel plate. In addition, the addition of V can combine with hydrogen (H) to improve the steel plate's resistance to delayed fracture. Furthermore, a large amount of V precipitates are retained within the steel plate microstructure as hydrogen traps, reducing the risk of delayed cracking during use. This results in excellent mechanical properties, porosity, and resistance to hydrogen-induced cracking. Higher V content significantly deteriorates the low-temperature toughness of the steel plate, and also worsens the toughness of the weld heat-affected zone. Therefore, the optimal V content range in this invention is between 0.35% and 0.55%, satisfying a C / V ratio of 0.45 to 0.7. The main purpose is to refine the grain and regulate strengthening: vanadium combines with carbon to form carbides such as VC, which refines the steel grain and improves strength and toughness. However, excessively high vanadium content can cause carbides to aggregate and grow, reducing toughness and increasing costs. Furthermore, it avoids deterioration of process performance: excessive vanadium will reduce the hot working properties of steel such as forging and rolling, and make it prone to cracking; at the same time, it will increase the hardenability fluctuation of steel and affect the stability of heat treatment. Limiting the C / V content can ensure the stability of processing and heat treatment. In order to achieve the optimal balance in terms of strength, toughness and processing performance of the steel of this invention, it can meet the application requirements.

[0017] Ti: Titanium effectively delays the recrystallization of deformed austenite, inhibits austenite grain growth, increases the austenite recrystallization temperature, refines the grain size, and improves the strength and toughness of steel. Furthermore, Ti is a strong carbide and nitride forming element, capable of combining with carbon and nitrogen to form stable and fine carbides and nitrides, resulting in significant grain refinement and precipitation strengthening. It can also strengthen ferrite and bainite, thus improving porosity. Therefore, the optimal Ti content range in this invention is between 0.075% and 0.110%, satisfying a C / Ti ratio of 2.4 to 3.3. The main purpose is to fix carbon to protect corrosion-resistant elements: Titanium has a much higher affinity for carbon than chromium, preferentially forming TiC in steel, reducing the combination of carbon and chromium, thereby protecting chromium and maintaining the integrity of the oxide film (especially in the weld heat-affected zone, preventing intergranular corrosion). It also refines the grain size and controls the microstructure: Titanium carbides (TiC) inhibit austenite grain growth, refine the steel grain size, and improve strength and toughness. However, excessive titanium can cause carbides to aggregate, which leads to a decrease in toughness and increases smelting costs. Therefore, the C / Ti ratio needs to be limited in order to achieve the optimal balance of strength, toughness, processing performance and corrosion resistance of the steel of this invention to meet the application requirements.

[0018] Chromium (Cr): Chromium is a carbide-forming element that can delay the pearlite transformation and improve the hardenability of steel. This promotes the formation of martensite and refines the microstructure, resulting in a strengthening effect. It also enhances the stability of austenite, inhibiting the transformation of austenite to other phases during coiling and improving the stability of residual austenite after coiling, ensuring a certain amount of residual austenite in the final microstructure. However, excessive chromium content can worsen the material's machinability and formability. The principle for selecting chromium content is to promote martensite formation. Chromium easily forms a dense and chemically stable oxide film (Cr2O3) on the steel surface. This film is not easily dissolved or destroyed in alkaline environments, effectively preventing contact between alkaline solutions and the steel matrix, fundamentally slowing down corrosion. In high-temperature or concentrated alkaline environments, chromium can inhibit the formation of harmful compounds (such as carbides) between carbon and other elements in steel, reducing intergranular corrosion and maintaining the integrity of the steel's microstructure. Chromium can increase the electrochemical potential of steel in alkaline media, reducing the corrosion rate, especially in strongly alkaline (such as sodium hydroxide) and high-temperature alkaline environments. Therefore, the chromium content of the present invention is selected in the range of 0.40% to 0.65%.

[0019] Ni: Nickel is a solid solution strengthening element that can improve the hardenability of materials, prevent temper brittleness, and improve the fatigue performance of materials. It has no adverse effect on the hardening and toughness of the weld heat-affected zone of steel, and improves heat resistance and cold brittleness. However, excessive Ni will lead to increased manufacturing costs. Nickel can promote the formation of a dense and stable oxide film (such as NiO) on the steel surface. This film is not easily destroyed in alkaline environments and can effectively block the contact between alkaline solutions and the steel matrix, reducing corrosion. Nickel can reduce the chemical reactivity of steel in concentrated alkali or high-temperature alkaline solutions, especially significantly inhibiting intergranular corrosion and stress corrosion caused by strong alkalis such as sodium hydroxide, maintaining the stability of the steel structure. Therefore, the optimal range of Ni content in this invention is between 0.150% and 0.190%.

[0020] Cu (Cu) plays a role in solid solution strengthening and precipitation strengthening. However, when the Cu content is too high, it can easily lead to copper embrittlement on the steel plate surface and the risk of hot-rolling edge cracking. In addition, Cu can reduce work hardening, improve the plasticity of the steel plate, and in an alkaline environment, copper will form a protective film containing copper oxide (CuO) or basic copper salts on the steel surface. This film has a relatively dense structure, which can slow down the erosion rate of alkaline solution on the steel substrate. Copper can reduce the electrochemical corrosion activity of steel in alkaline media, especially in medium-low temperature and medium concentration alkaline environments, it can inhibit uniform corrosion and localized corrosion (such as pitting corrosion) of steel, and enhance overall corrosion resistance. Therefore, the optimal range of Cu content in this invention is between 0.160% and 0.200%.

[0021] Tantalum (Ta) has a strong affinity for elements such as carbon and nitrogen in steel, forming fine and dispersed carbides, nitrides, or carbonitrides. These compounds hinder grain growth, refining the steel's grain structure and thus improving its overall mechanical properties, including strength, toughness, and plasticity, as well as its porosity. Furthermore, tantalum itself exhibits extremely high stability in strongly alkaline environments (such as sodium hydroxide solution) from room temperature to high temperatures, resisting chemical reactions with alkalis and directly resisting their corrosion. Tantalum readily forms a dense and chemically stable oxide film (Ta₂O₅) on the steel surface. This film is difficult to dissolve in alkaline media, effectively preventing contact between the alkali solution and the substrate, protecting the steel from corrosion. Therefore, the optimal Ta content in this invention is between 0.10% and 0.32%.

[0022] Y: Yttrium can refine the grains in steel, enhance the strength and plasticity of grain boundaries, improve the hole expansion performance, and also improve the weldability and oxidation resistance of steel, thereby increasing the service life of steel at high temperatures. Therefore, the Y content is limited to 0.003% to 0.010% in this invention.

[0023] 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 VN formed with V to carry out precipitation strengthening and grain refinement strengthening, thereby improving the strength and pore-expanding performance of the steel plate. Therefore, the N content in this invention is ≤0.006%.

[0024] A method for producing ultra-high strength, high elongation hot-rolled pickled steel sheet for automotive applications includes smelting, continuous casting, hot rolling, and pickling processes, specifically comprising:

[0025] (1) Smelting process: The raw materials are pretreated with KR hot metal to control the S content to be less than 0.005%. After slag removal, they enter the converter. In the converter smelting, the double slag method is used to remove P, and the P content is controlled to be ≤0.015%. At the end of the converter smelting, the C content is controlled to be 0.240-0.270%. Argon gas is blown for more than 5 minutes when tapping the steel (the argon blowing and calming before continuous casting can promote the removal of inclusions in the molten steel and improve the uniformity of the steel composition). Then, the LF+RH process is carried out, and the H and O contents are strictly controlled, H≤0.0002% and O≤0.0015%. The RH vacuum degassing is maintained for more than 18 minutes. Calcium treatment is carried out in the refining LF process, and then the slab is continuously cast. The superheat of continuous casting is 20-25℃, and the continuous casting drawing speed is 0.8-1.1m / min (reducing the temperature). Lower superheat and reduced continuous casting speed can improve macroscopic segregation of the continuously cast billet, reduce the spacing of secondary dendrite arms in the solidification structure of the billet, and help reduce billet segregation and internal structural defects. The electromagnetic stirring current intensity in the secondary cooling zone during the continuous casting stage is controlled at 260A to 360A, and the secondary cooling water volume is 0.85L / kg to 1.15L / kg (to reduce the average carbon segregation index, suppress segregation, and limit the intensity of secondary cooling to suppress the tendency of central cracks in the billet to worsen). In the horizontal section of the secondary cooling zone, i.e., at the end of solidification, a light pressure is applied, and the billet reduction is 3.0 to 4.5mm (to reduce the central porosity and segregation of the billet). The billets are stacked for more than 36 hours after being removed from the line (to reduce the accumulation of residual H, suppress the generation of microcracks inside the billet, and ensure the toughness of the steel plate).

[0026] (2) In the hot rolling process: a continuously cast slab (155-245) mm thick × (1040-2000) mm wide is loaded into a walking beam furnace for heating at a temperature of 1235-1280℃ and a holding time of 160-180 min. The rough rolling adopts a 3+3 rolling process (R1 is rolled in 3 passes, R2 is rolled in 3 passes), for a total of 6 passes. The exit temperature of the rough rolling is 1020-1080℃. The billet thickness is 38.6–55.5 mm, and the width is 1040–2000 mm. The intermediate billets are insulated with heat shields before entering the hot finishing mill to reduce temperature drop on the delay roller table and temperature differences at the head and tail and in the width direction. Finishing is a 7-stand continuous rolling process. High-pressure water descaling is performed before finishing. The finishing mill inlet temperature is 990–1050℃, and the final rolling temperature is 830–898℃. After finishing, a laminar flow cooling + air cooling + rapid cooling mode is used. The cooling rate is greater than 18℃ / s. After cooling to 645-675℃, air cooling is performed. After air cooling for 8-14 seconds, rapid cooling is performed at a rate greater than 108℃ / s. After rapid cooling to 260-320℃, the coil is coiled. Immediately after coiling, the coil is placed in a heated and insulated pit at a temperature of 360-380℃ and held for 20-40 minutes. The coil is then removed and air-cooled to room temperature. Laminar flow cooling to 645-675℃ followed by air cooling for 8-14 seconds aims to rapidly generate ferrite or bainite, inhibiting grain growth while ensuring the ferrite or bainite content, thus refining the ferrite or bainite grains. Rapid cooling to 260-320℃ aims to cool to the martensitic region, exceeding the critical cooling rate for pearlite transformation, and avoiding the pearlite formation region. This inhibits grain growth while ensuring the martensitic content, thus refining the martensitic grains. Immediately after coiling, the steel coil is placed in a heated and insulated pit at 360–380℃ for 20–40 minutes. Afterward, it is removed and air-cooled to room temperature. This process allows carbon from the martensite to diffuse into the retained austenite, improving austenite stability, increasing the percentage of retained austenite, and thus enhancing the plasticity of the steel plate. The final microstructure of the rolled steel plate consists of 0%–8% ferrite, 16%–19% bainite, 73%–82% martensite, and 1%–4% retained austenite.

[0027] (3) The pickling process: Hydrochloric acid is used to continuously clean the iron oxide scale of the steel plate through the coil. After hot rolling, the steel plate is uncoiled and pickled on the pickling line. Before pickling, the steel plate is tension leveled with an elongation of 0.7-1.2%. The pickling solution is hydrochloric acid. The pickling tank is divided into 4 tanks. The concentration of the pickling solution in tank 1 is 50-70 g / L and the temperature of the pickling solution in tank 1 is 77.0-86.0℃. The concentration of the pickling solution in tank 2 is 72-103 g / L and the temperature of the pickling solution in tank 2 is 68.0-77.0℃. The concentration of the pickling solution in tank 3 is 120-140 g / L and the temperature of the pickling solution in tank 3 is 63.0-68.0℃. The concentration of the pickling solution in tank 4 is 145-160 g / L and the temperature of the pickling solution in tank 4 is 55.0-63.0℃. During pickling, a corrosion inhibitor is added to the acid solution, with the corrosion inhibitor accounting for 0.09–0.14% of the pickling solution by weight. The rinsing water temperature is 52–62℃, and the pickling and rinsing speeds are controlled at 70–100 m / min. The pickling process is carried out under tension, with a tension of 40–55 kN. Finally, the product is coated with oil and rolled up to obtain the finished product, which has a thickness of 1.50–6.00 mm and a surface roughness Ra of 0.78–1.40 μm.

[0028] Compared with the prior art, the beneficial effects of the present invention are:

[0029] 1. The addition of vanadium and the VN precipitation formed by nitrogen can greatly improve the strength of the steel plate. In addition, the addition of V can also combine with H to improve the steel plate's resistance to delayed fracture. Furthermore, a large number of V precipitates are retained in the steel plate structure as hydrogen traps, reducing the risk of delayed cracking in the steel plate of this invention during use. It can obtain hot-rolled steel plates with excellent mechanical properties, hole expansion properties and resistance to hydrogen-induced cracking.

[0030] 2. The addition of chromium can delay the pearlite transformation and improve the hardenability of steel. This is conducive to the formation of martensite and refines the microstructure, resulting in a strengthening effect. It also enhances the stability of austenite, inhibiting the transformation of austenite to other phases during coiling and improving the stability of residual austenite after coiling, ensuring a certain amount of residual austenite in the final microstructure. Furthermore, chromium easily forms a dense and chemically stable oxide film (Cr2O3) on the steel surface. This film is not easily dissolved or destroyed in alkaline environments, effectively preventing contact between alkaline solutions and the steel matrix, fundamentally slowing down corrosion. In high-temperature or concentrated alkaline environments, chromium can inhibit the formation of harmful compounds (such as carbides) between carbon and other elements in steel, reducing intergranular corrosion and maintaining the integrity of the steel's microstructure. Chromium can increase the electrochemical potential of steel in alkaline media, reducing the corrosion rate of steel.

[0031] 3. Nickel can improve the hardenability of materials, prevent temper brittleness, and improve the fatigue performance of materials. It has no adverse effect on the hardening properties and toughness of the weld heat-affected zone of steel, and improves heat resistance and cold brittleness. Furthermore, nickel can promote the formation of a dense and stable oxide film (such as NiO) on the steel surface. This film is not easily destroyed in alkaline environments and can effectively prevent the contact between alkaline solutions and the steel substrate, reducing corrosion. Nickel can reduce the chemical reactivity of steel in concentrated alkalis or high-temperature alkaline solutions, especially significantly inhibiting intergranular corrosion and stress corrosion caused by strong alkalis such as sodium hydroxide, thus maintaining the stability of the steel structure.

[0032] 4. Copper plays a role in solid solution strengthening and precipitation strengthening. Simultaneously, it significantly improves the corrosion resistance of steel plates, reduces work hardening, and enhances the plasticity of the steel. Furthermore, in alkaline environments, copper forms a protective film containing copper oxide (CuO) or basic copper salts on the steel surface. This film has a relatively dense structure, which slows down the erosion rate of alkaline solutions on the steel substrate. Copper can reduce the electrochemical corrosion activity of steel in alkaline media, especially in medium- and low-temperature, medium-concentration alkaline environments, inhibiting uniform and localized corrosion (such as pitting corrosion) and enhancing overall corrosion resistance.

[0033] 5. Tantalum has a strong affinity for elements such as carbon and nitrogen in steel, forming fine and dispersed carbides, nitrides, or carbonitrides. These compounds hinder grain growth, refining the steel's grain structure and thus improving its overall mechanical properties, including strength, toughness, and plasticity, as well as its porosity. Furthermore, tantalum itself exhibits extremely high stability in strongly alkaline environments (such as sodium hydroxide solution) from room temperature to higher temperatures, resisting chemical reactions with alkalis and directly resisting their corrosion. Tantalum readily forms a dense and chemically stable oxide film (Ta₂O₅) on the steel surface. This film is difficult to dissolve in alkaline media, effectively preventing contact between the alkali solution and the substrate, protecting the steel from corrosion.

[0034] 6. The addition of Y can refine the grains in steel, enhance the strength and plasticity of grain boundaries, improve the hole expansion performance, and also improve the weldability and oxidation resistance of steel, thereby increasing the service life of steel at high temperatures.

[0035] 7. The steel of this invention contains 0%–8% ferrite, 16%–19% bainite, 73%–82% martensite, and 1%–4% retained austenite, thereby significantly improving the mechanical properties and hole-expanding performance of the steel plate during the forming process.

[0036] 8. This invention possesses excellent mechanical properties: yield strength ≥735MPa, tensile strength ≥1155MPa, longitudinal elongation A ≥28%, porosity ≥46%, longitudinal cold bending at 180° D=a is qualified, and surface roughness Ra is 0.78~1.40μm. Using an alkaline solution of 3.7g / L NaOH + 10.5g / L KOH + 2g / L Ca(OH)2, the corrosion rate is ≤0.41g / (m²) within a 168h test period. 2 ·h). Detailed Implementation

[0037] 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.

[0038] The embodiments of the invention are as follows:

[0039] The smelting raw materials undergo KR hot metal pretreatment to control the sulfur content below 0.005%. After slag removal, the material enters the converter. During converter smelting, a double-slag method is used to remove phosphorus (P), controlling the P content to ≤0.015%. At the end of converter smelting, the carbon content is controlled between 0.240% and 0.270%. Argon gas is blown for at least 5 minutes during tapping, followed by the LF+RH process. The H and O contents are strictly controlled: H ≤0.0002%, O ≤0.0015%. RH vacuum degassing is maintained for at least 18 minutes. In the refining LF process, calcium treatment is carried out, followed by slab continuous casting. The superheat of continuous casting is 20-25℃, the casting speed is 0.8-1.1m / min, the electromagnetic stirring current intensity in the secondary cooling zone is controlled at 260A-360A, the secondary cooling water volume is 0.85L / kg-1.15L / kg, and light pressure is applied in the horizontal section of the secondary cooling zone, i.e., at the end of solidification, the reduction of the continuous casting slab is 3.0-4.5mm, and the slabs are stacked for more than 36 hours after leaving the line. A continuously cast slab with a thickness of (155–245) mm and a width of (1040–2000) mm is loaded into a walking beam furnace for heating at a temperature of 1235–1280℃ for 160–180 min. The roughing process uses a 3+3 rolling mode (R1 is rolled in 3 passes, R2 in 3 passes), for a total of 6 passes. The exit temperature of the roughing mill is 1020–1080℃. The intermediate slab has a thickness of 38.6–55.5 mm and a width of 1040–2000 mm. Before entering the hot finishing mill, the intermediate slab is kept warm. A heat shield is used to reduce the temperature drop of the intermediate billet on the delay roller table and the temperature difference between the head and tail and the width of the plate. Finishing rolling is a 7-stand continuous rolling process. High-pressure water descaling is performed before finishing rolling. The entry temperature of the finishing mill is 990–1050℃, and the final rolling temperature is 830–898℃. After finishing rolling, a laminar flow cooling + air cooling + rapid cooling mode is adopted. The laminar flow cooling rate is greater than 18℃ / s. After cooling to 645–675℃, air cooling is performed. After 8–14 seconds of air cooling, rapid cooling is performed at a rate greater than 108℃ / s. After rapid cooling to 260–320℃, the coil is coiled. Immediately after coiling, it enters a heated holding pit with a heating temperature of 360–380℃ and is held for 20–40 minutes. The coil is then removed and air-cooled to room temperature. After hot rolling, the steel plate is uncoiled on the pickling line and pickled with hydrochloric acid. Before pickling, the steel plate is tension-straightened with an elongation of 0.7-1.2%. The pickling solution is hydrochloric acid, and the pickling tank is divided into 4 tanks. The concentration of pickling solution in tank 1 is 50-70 g / L, and the temperature of pickling solution in tank 1 is 77.0-86.0℃. The concentration of pickling solution in tank 2 is 72-103 g / L, and the temperature of pickling solution in tank 2 is 68.0-77.0℃. The concentration of pickling solution in tank 3 is 120-140 g / L, and the temperature of pickling solution in tank 3 is 63.0-68.0℃. The concentration of pickling solution in tank 4 is 145-160 g / L, and the temperature of pickling solution in tank 4 is 55.0-63.0℃.During pickling, a corrosion inhibitor is added to the acid solution, with the inhibitor accounting for 0.09% to 0.14% of the pickling solution by weight. The rinsing water temperature is 52 to 62°C, and the pickling and rinsing speeds are controlled at 70 to 100 m / min. The pickling process is carried out under tension, with a tension of 40 to 55 kN. Finally, the product is coated with oil and rolled up to obtain the finished product. The finished product thickness is 1.50 to 6.00 mm, the surface roughness Ra is 0.78 to 1.40 μm, the surface quality is good, and there are no iron oxide scale streaks or color difference defects.

[0040] The specific components, smelting process, hot rolling process, pickling process, steel plate properties, and volume percentage of the six embodiments of the present invention are shown in Tables 1-6.

[0041] Table 1 Chemical composition (wt, %) of embodiments of the present invention

[0042]

[0043] Table 2 Smelting process of embodiments of the present invention

[0044]

[0045] Table 3 Hot rolling process of the present invention embodiments

[0046]

[0047] Table 4. Pickling process of the present invention embodiments

[0048]

[0049] Table 5 Mechanical performance parameters of embodiments of the present invention

[0050]

[0051] Table 6. Percentage of tissue volume in embodiments of the present invention

[0052]

Claims

1. A hot-rolled pickled steel sheet for automotive applications with ultra-high strength and high elongation, characterized in that, The chemical composition of the steel, by weight percentage, is as follows: C: 0.240%–0.270%, Si: 0.40%–0.62%, Mn: 1.85%–2.60%, Al: 0.45%–0.66%, V: 0.35%–0.55%, Ti: 0.075%–0.110%, Cr: 0.40%–0.65%, Ni: 0.150%–0.190%, Cu: 0.160%–0.200%, Ta: 0.10%–0.32%, Y: 0.003%–0.010%, with Al / Si ratios of 0.8–1.5, C / V ratios of 0.45–0.7, and C / Ti ratios of 2.4–3.

3. P is limited to ≤0.015%, S to ≤0.005%, and N to ≤0.006%, with the balance being Fe and unavoidable impurities. The microstructure of the finished steel plate consists of 0-8% ferrite, 16-19% bainite, 73-82% martensite, and 1-4% retained austenite. The production method of ultra-high strength and high elongation hot-rolled pickled steel sheet for automobiles includes smelting, continuous casting, hot rolling, and pickling processes, specifically including: In the hot rolling process: the heating temperature is 1235-1280℃, the holding time is 160-180min, the roughing mill exit temperature is 1020-1080℃, the finishing mill inlet temperature is 990-1050℃, and the final rolling temperature is 830-898℃. After final rolling, a laminar flow cooling + air cooling + rapid cooling mode is adopted. The laminar flow cooling rate is greater than 18℃ / s. After cooling to 645-675℃, air cooling is performed. After air cooling for 8-14s, rapid cooling is performed. The rapid cooling rate is greater than 108℃ / s. After rapid cooling to 260-320℃, the coil is coiled. After coiling, it is immediately placed in a heated holding pit with a heating temperature of 360-380℃ and held for 20-40min. The coil is then removed and air-cooled to room temperature.

2. The ultra-high strength, high elongation hot-rolled pickled steel sheet for automobiles according to claim 1, characterized in that, The steel plate has a yield strength ≥735MPa, tensile strength ≥1155MPa, longitudinal elongation A ≥28%, hole expansion rate ≥46%, and longitudinal cold bending 180° D=a is qualified.

3. The ultra-high strength, high elongation hot-rolled pickled steel sheet for automobiles according to claim 1, characterized in that, The finished steel plate has a thickness of 1.50–6.00 mm and a surface roughness Ra of 0.78–1.40 μm. Using an alkaline solution of 3.7 g / L NaOH + 10.5 g / L KOH + 2 g / L Ca(OH)₂, the corrosion rate of the steel plate is ≤0.41 g / (m²) over a 168-hour test period. 2 ·h).

4. A method for producing ultra-high strength, high elongation hot-rolled pickled steel sheet for automobiles as described in any one of claims 1-3, characterized in that, It includes smelting, continuous casting, hot rolling, and pickling processes, specifically: In the hot rolling process: the heating temperature is 1235-1280℃, the holding time is 160-180min, the roughing mill exit temperature is 1020-1080℃, the finishing mill inlet temperature is 990-1050℃, and the final rolling temperature is 830-898℃. After final rolling, a laminar flow cooling + air cooling + rapid cooling mode is adopted. The laminar flow cooling rate is greater than 18℃ / s. After cooling to 645-675℃, air cooling is performed. After air cooling for 8-14s, rapid cooling is performed. The rapid cooling rate is greater than 108℃ / s. After rapid cooling to 260-320℃, the coil is coiled. After coiling, it is immediately placed in a heated holding pit with a heating temperature of 360-380℃ and held for 20-40min. The coil is then removed and air-cooled to room temperature.

5. The method for producing ultra-high strength, high elongation hot-rolled pickled steel sheet for automobiles according to claim 4, characterized in that, In the smelting process: Argon gas is blown for more than 5 minutes when the converter taps out steel, and RH vacuum degassing is maintained for more than 18 minutes.

6. The method for producing ultra-high strength, high elongation hot-rolled pickled steel sheet for automobiles according to claim 4, characterized in that, In the continuous casting process: the superheat of continuous casting is 20-25℃, the continuous casting billet pulling speed is 0.8-1.1m / min, the electromagnetic stirring current intensity in the secondary cooling zone of the continuous casting stage is controlled at 260-360A, the secondary cooling water volume is 0.85-1.15L / kg, light pressure is applied at the end of solidification, the reduction of the continuous casting billet is 3.0-4.5mm, and the billets are stacked for more than 36 hours after leaving the line.

7. The method for producing ultra-high strength, high elongation hot-rolled pickled steel sheet for automobiles according to claim 4, characterized in that, The specifications of the continuously cast billet are (155~245)mm thick × (1040~2000)mm wide. The intermediate billet between roughing and finishing rolling has a thickness of 38.6~55.5mm and a width of 1040~2000mm.

8. The method for producing ultra-high strength, high elongation hot-rolled pickled steel sheet for automobiles according to claim 4, characterized in that, The pickling process involves the following steps: Before pickling, the steel plate is tension-straightened with an elongation rate of 0.7%–1.2%. The pickling tank is divided into four tanks: Tank 1 has a concentration of 50–70 g / L and a pickling solution temperature of 77.0–86.0℃; Tank 2 has a concentration of 72–103 g / L and a pickling solution temperature of 68.0–77.0℃; and Tank 3 has a concentration of 120–140 g / L and a pickling solution temperature of 63.0–68℃. The pickling solution in tank 4 is 55.0-63.0℃, with a concentration of 145-160 g / L. During pickling, a corrosion inhibitor is added to the acid solution, accounting for 0.09%-0.14% of the weight of the pickling solution. The rinsing water temperature is 52-62℃, and the pickling and rinsing speeds are controlled at 70-100 m / min. The pickling process is under tension, with a tension of 40-55 kN. The pickling solution is hydrochloric acid.

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