High-formability hot-rolled pickled steel sheet for automobiles and method for producing the same

Abstract

The present application belongs to the field of metal materials, and particularly relates to a high-forming hot-rolled pickling steel plate for automobiles 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, Nb, Ti, Cr, Ni, Cu, Ta, Zr, Y, and Al / Si: 1.0-1.9, Ta+Zr: 0.017%-0.023%, and P≤0.015%, S≤0.005%, N≤0.003% are limited, and the balance is Fe and inevitable impurities. The present application has excellent mechanical properties, the yield strength is ≥590 MPa, the tensile strength is ≥890 MPa, the longitudinal elongation A is ≥32%, the hole expansion ratio is ≥52%, the longitudinal cold bending 180° D=a is qualified, the surface roughness Ra is 0.75-1.38 μm, and the corrosion rate of the steel plate is ≤0.42 g / (m 2 ·h).

Description

Technical Field

[0001] This invention belongs to the field of metallic materials, and specifically relates to a high-formability hot-rolled pickled steel sheet for automobiles and its production method. The steel sheet of this invention is mainly suitable for manufacturing automobile chassis structural parts. 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 CN108277434A discloses a precipitation-strengthened ferritic steel with a yield strength of 900 MPa and its production method. The process involves conventional smelting, casting into a billet, heating the billet, two-stage hot rolling, laminar flow cooling, coiling, natural cooling to room temperature followed by pickling, warm rolling, annealing, and cooling. The steel's yield strength is 903–946 MPa, tensile strength is 952–987 MPa, and elongation is ≥21%, which does not meet the requirements for high-formability automotive chassis structural components and also lacks sufficient corrosion resistance and oxidation resistance.

[0004] Chinese patent application CN110117759B discloses a manufacturing process for austenitic stainless steel used in roll forming of high-strength steel structural components. The process includes smelting and casting, forging into billets, hot rolling, hot rolling annealing and pickling, cold rolling, low-temperature annealing, and air cooling. This invention has low raw material costs; the resulting austenitic stainless steel exhibits good corrosion resistance, with a yield strength exceeding 600 MPa, a tensile strength exceeding 900 MPa, and an elongation exceeding 35%. However, this steel plate is a cold-rolled steel plate, and the relevant properties require further cold rolling and annealing processes to obtain, and it lacks certain corrosion resistance and oxidation resistance. Summary of the Invention

[0005] To address the development needs of the automotive steel industry, this invention provides a high-formability hot-rolled pickled steel sheet for automobiles and its production method. The sheet exhibits a yield strength ≥ 590 MPa, tensile strength ≥ 890 MPa, longitudinal elongation A ≥ 32%, hole expansion rate ≥ 52%, and meets the acceptable longitudinal cold bending requirement of 180° (D=a). The surface roughness Ra is 0.75–1.38 μm, and it possesses excellent corrosion resistance. 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 sheet is ≤ 0.42 g / (m²) over a 168-hour test period. 2 ·h).

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

[0007] A high-formability hot-rolled pickled steel sheet for automobiles, the chemical composition of which, by weight percentage, is: C: 0.190%–0.240%, Si: 0.25%–0.44%, Mn: 1.80%–2.50%, Al: 0.35%–0.55%, Nb: 0.045%–0.065%, Ti: 0.025%–0.045%, Cr: 0.33%–0.52%, Ni: 0.100%–0. 0.140%, Cu: 0.150%~0.190%, Ta: 0.007%~0.012%, Zr: 0.008%~0.013%, Y: 0.004%~0.009%, and Al / Si: 1.0~1.9, Ta+Zr: 0.017%~0.023%, with P≤0.015%, S≤0.005%, N≤0.003%, and the balance being Fe and unavoidable impurities.

[0008] The finished steel plate of this invention has a ferrite volume percentage of 8% to 12%, a bainite volume percentage of 21% to 31%, a martensite volume percentage of 55% to 62%, and a retained austenite volume percentage of 2% to 5%.

[0009] The steel plate of this invention has a yield strength ≥590MPa, tensile strength ≥890MPa, longitudinal elongation A ≥32%, hole expansion rate ≥52%, and longitudinal cold bending 180° D=a is qualified.

[0010] The finished steel plate of this invention has a thickness of 1.50–6.00 mm and a surface roughness Ra of 0.75–1.38 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 ≤0.42 g / (m²) over a 168-hour test period. 2 ·h).

[0011] The main function of the chemical composition of a high-formability hot-rolled pickled steel sheet for automobiles in this invention is as follows:

[0012] 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 0.190% to 0.240%.

[0013] 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 0.25%–0.44%.

[0014] 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 expansion performance of the steel plate. Furthermore, the added manganese content should not exceed the scope of this invention. The main consideration is that excessive manganese content will lead to C or Mn segregation, which will worsen the uniformity of the steel plate structure during hot rolling and easily cause severe banded defects in the structure. In addition, excessive manganese in the steel involved in this invention will increase hardenability, inhibit bainite formation, and is also not conducive to hole expansion 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 1.80% to 2.50%.

[0015] 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%.

[0016] 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%.

[0017] 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 grains. 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 0.35%–0.55%, and the Al / Si ratio is set at 1.0–1.9. 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.

[0018] Niobium (Nb) effectively delays the recrystallization of deformed austenite, inhibits austenite grain growth, increases the austenite recrystallization temperature, refines grains, and improves the strength and toughness of steel. Since free N atoms in steel deteriorate the toughness of the steel plate, Nb combines with impurity N elements in the steel to form NbN, thus NbN formation has a solidification effect. Furthermore, Nb also combines with C and N to form Nb(C,N), playing a role in grain refinement and precipitation strengthening. It can also strengthen ferrite and bainite, but excessive Nb content will lead to excessively large NbN sizes, deteriorating the steel plate properties and reducing the toughness of the weld heat-affected zone. To obtain excellent mechanical properties and hole-expanding performance, the optimal range of Nb content in this invention is between 0.045% and 0.065%.

[0019] Ti: Titanium has significant precipitation strengthening and grain refinement strengthening effects. Its effect is mainly achieved through the formation of precipitates with carbon and nitrogen, especially TiN precipitation with nitrogen, which can greatly improve the strength of the steel plate. In addition, the retention of a large amount of Ti precipitates within the steel plate microstructure acts as a hydrogen trap, reducing the risk of delayed cracking during use and achieving excellent mechanical properties and resistance to hydrogen-induced cracking. When the Ti content is high, the toughness of the weld heat-affected zone also deteriorates; therefore, the Ti addition amount in this invention is 0.025%–0.045%.

[0020] 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 0.33% to 0.52%.

[0021] 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 substrate, 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 0.100% and 0.140%.

[0022] 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 forms 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 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, 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 0.150% and 0.190%.

[0023] 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 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 0.007% and 0.012%.

[0024] Zirconium (Zr) is a strong deoxidizer that can react with oxygen to form stable ZrO2, effectively reducing the oxygen content in steel, reducing oxide inclusions, and improving the purity of steel. It can also react with sulfur to form ZrS, reducing the sulfur content in steel. During steel solidification, it can act as a heterogeneous nucleus, promoting grain nucleation, inhibiting grain growth, and refining as-cast grains. Furthermore, during hot rolling, it forms stable carbides, hindering austenite grain growth, resulting in a fine and uniform grain structure, and improving the strength, toughness, plasticity, and porosity of steel. Therefore, this invention limits the Zr content to 0.008% to 0.013%, and satisfies Ta + Zr: 0.017% to 0.023%. The main purpose is to synergistically optimize the high-temperature performance and microstructure stability of steel, avoiding performance deterioration due to improper content. Both Ta and Zr are strong carbide-forming elements. When Zr is combined with Ta, it can help refine the carbide distribution to avoid excessive carbide precipitation or aggregation, enhance microstructure stability at high temperatures, and control the total alloy content to balance cost and performance.

[0025] 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 0.004% to 0.009% in this invention.

[0026] N: For the N content in steel, the lower the N content, the better, but too low a content will lead to production difficulties and increased costs. Therefore, the N content in this invention is ≤0.003%.

[0027] A method for producing high-formability hot-rolled pickled steel sheet for automobiles includes smelting, continuous casting, hot rolling, and pickling processes, specifically comprising:

[0028] (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 0.190% to 0.240%. Argon gas is blown for more than 3 minutes when tapping the steel (the argon blowing and sedation 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 15 minutes. Calcium treatment is carried out in the refining LF process. After that, slab continuous casting is carried out. The superheat of continuous casting is 15 to 20℃, and the continuous casting drawing speed is 0.8 to 1.1 m / min. (Reducing superheat and casting speed can improve macroscopic segregation of the billet, reduce the spacing of secondary dendrite arms in the solidification structure of the billet, and help reduce billet segregation and internal structural defects.) Control the electromagnetic stirring current intensity in the secondary cooling zone of the continuous casting stage to 280A~380A, and the secondary cooling water volume to 0.85~1.15L / 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 3.0~4.5mm (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).

[0029] (2) In the hot rolling process: a continuously cast slab with a thickness of (170-235) mm and a width of (1040-2000 mm) is loaded into a walking beam furnace for heating at a temperature of 1235-1250℃ and a holding time of 150-180 min. The roughing process adopts a 3+3 rolling process (R1 is rolled in 3 passes and R2 is rolled in 3 passes) for a total of 6 passes. The exit temperature of the roughing mill is 1020-1080℃. The thickness of the intermediate slab is 37.0-55.0 mm and the width is 1040-2000 mm. The intermediate slab is kept warm by an insulation cover before entering the hot finishing mill to reduce the temperature drop of the intermediate slab on the delay roller table and the head. Temperature difference in the tail and width directions: Finishing rolling is a 7-stand continuous rolling process. High-pressure water descaling is performed before finishing rolling. The entry temperature of finishing rolling is 990-1050℃, and the finishing rolling temperature is 820-890℃. After finishing rolling, laminar flow cooling + air cooling + rapid cooling mode is adopted. The laminar flow cooling rate is greater than 16℃ / s. After cooling to 765-835℃, air cooling is performed. After air cooling for 15-20s, rapid cooling is performed. The rapid cooling rate is greater than 105℃ / s. After rapid cooling to 290-350℃, coiling is performed. After coiling, it is immediately placed in a heated and heat-preserving pit with a heating temperature of 360-380℃ and held for 20-40min. The steel coil is then removed and air-cooled to room temperature. After laminar flow cooling to 765–835℃, air cooling for 15–20 seconds is performed to rapidly generate ferrite, inhibiting grain growth while ensuring ferrite content and thus refining the ferrite grains. Rapid cooling to 290–350℃ is then performed to cool to the martensitic region, exceeding the critical cooling rate for pearlite transformation, avoiding the pearlite formation zone. This again inhibits grain growth while ensuring martensite content and refining the martensite grains. Immediately after coiling, the coil is placed in a heated holding pit at 360–380℃ for 20–40 minutes. Afterward, the coil is removed and air-cooled to room temperature. This allows carbon from the martensite to diffuse into the retained austenite, improving austenite stability, increasing the percentage of retained austenite, and thus improving the plasticity of the steel plate. The final microstructure of the rolled steel plate consists of 8%–12% ferrite, 21%–31% bainite, 55%–62% martensite, and 2%–5% retained austenite.

[0030] (3) The pickling process includes: continuously cleaning the iron oxide scale of the steel plate with hydrochloric acid; after hot rolling, the steel plate is uncoiled and pickled on the pickling line; before pickling, the steel plate is tension-straightened with an elongation of 0.7% to 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 to 70 g / L, and the temperature of the pickling solution in tank 1 is 77.0 to 86.0℃; the concentration of the pickling solution in tank 2 is 72 to 103 g / L, and the temperature of the pickling solution in tank 2 is 68.0 to 77.0℃; the concentration of the pickling solution in tank 3 is 120 to 140 g / L, and the temperature of the pickling solution in tank 3 is 63.0℃. The pickling temperature 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 total weight. The rinsing water temperature is 52–62℃, and the pickling and rinsing speeds are controlled at 80–120 m / min. The pickling process is conducted under tension, with a tension of 40–55 kN. Finally, the product is oiled and rolled up to obtain a finished product with a thickness of 1.50–6.00 mm and a surface roughness Ra of 0.75–1.38 μm.

[0031] Compared with the prior art, the beneficial effects of the present invention are:

[0032] 1. The addition of titanium and nitrogen forms TiN precipitation, which can improve the strength of the steel plate and retain a large amount of Ti precipitate phase inside the steel plate structure as a hydrogen trap, reducing the risk of delayed cracking in the steel plate during use, and enabling the steel plate to obtain excellent mechanical properties and resistance to hydrogen-induced cracking.

[0033] 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.

[0034] 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.

[0035] 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.

[0036] 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 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.

[0037] 6. Zirconium is a strong deoxidizer that can react with oxygen to form stable ZrO2, effectively reducing the oxygen content in steel, reducing oxide inclusions, and improving the purity of steel. It can also react with sulfur to form ZrS, reducing the sulfur content in steel. During steel solidification, it can act as a heterogeneous nucleus, promoting grain nucleation, inhibiting grain growth, and refining cast grains. Furthermore, during hot rolling, it forms stable carbides, hindering austenite grain growth, resulting in a fine and uniform grain structure, and improving the strength, toughness, plasticity, and porosity of steel.

[0038] 7. 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.

[0039] 8. The steel of this invention contains 8%–12% ferrite, 21%–31% bainite, 55%–62% martensite, and 2%–5% retained austenite, thereby significantly improving the mechanical properties and hole-expanding performance of the steel plate during the forming process.

[0040] 9. This invention possesses excellent mechanical properties: yield strength ≥ 590 MPa, tensile strength ≥ 890 MPa, longitudinal elongation A ≥ 32%, hole expansion rate ≥ 52%, longitudinal cold bending at 180° D=a is qualified, and surface roughness Ra is 0.75~1.38 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 ≤ 0.42 g / (m²) within a 168-hour test period. 2 ·h). Detailed Implementation

[0041] 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.

[0042] The embodiments of the invention are as follows:

[0043] 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 0.190% and 0.240%. Argon gas is blown for at least 3 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 15 minutes. In the refining LF process, calcium treatment is carried out, followed by slab continuous casting. The superheat of continuous casting is 15-20℃, the casting speed is 0.8-1.1m / min, the electromagnetic stirring current intensity in the secondary cooling zone is controlled at 280A-380A, 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 continuous casting billet reduction is 3.0-4.5mm, and the billets are stacked for more than 36 hours after leaving the line. A continuously cast slab with a thickness of (170–235) mm and a width of (1040–2000) mm is loaded into a walking beam furnace for heating at a temperature of 1235–1250℃ for 150–180 min. The roughing process uses a 3+3 rolling pattern (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 37.0–55.0 mm and a width of 1040–2000 mm. The intermediate slab is kept warm before entering the hot finishing mill. The steel is insulated 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 is a 7-stand continuous rolling process. High-pressure water descaling is performed before finishing. The entry temperature of the finishing mill is 990–1050℃, and the final rolling temperature is 820–890℃. After finishing, a laminar flow cooling + air cooling + rapid cooling mode is used. The laminar flow cooling rate is greater than 16℃ / s. After cooling to 765–835℃, air cooling is performed. After 15–20 seconds of air cooling, rapid cooling is performed at a rate greater than 105℃ / s. After rapid cooling to 290–350℃, the coil is coiled. Immediately after coiling, it enters a heated holding pit at 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 80 to 120 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 0.75 to 1.38 μm, the surface quality is good, and there are no iron oxide scale streaks or color difference defects.

[0044] 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.

[0045] Table 1 Chemical composition (wt, %) of embodiments of the present invention

[0046]

[0047] Table 2 Smelting process of embodiments of the present invention

[0048]

[0049] Table 3 Hot rolling process of the present invention embodiments

[0050]

[0051] Table 4. Pickling process of the present invention embodiments

[0052]

[0053] Table 5 Mechanical performance parameters of embodiments of the present invention

[0054]

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

[0056]

Claims

1. A high-formability hot-rolled pickled steel sheet for automobiles, characterized in that, The chemical composition of the steel, by weight percentage, is as follows: C: 0.190%–0.240%, Si: 0.25%–0.44%, Mn: 1.80%–2.50%, Al: 0.35%–0.55%, Nb: 0.045%–0.065%, Ti: 0.025%–0.045%, Cr: 0.33%–0.52%, Ni: 0.100%–0.140%, Cu The composition of the compounds is as follows: 0.150%–0.190%, Ta: 0.007%–0.012%, Zr: 0.008%–0.013%, Y: 0.004%–0.009%, with Al / Si ratio of 1.0–1.9, Ta+Zr: 0.017%–0.023%, and P ≤ 0.015%, S ≤ 0.005%, N ≤ 0.003%, with the balance being Fe and unavoidable impurities. The production method of high-formability 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 1235-1250℃, the holding time is 150-180min, the roughing mill exit temperature is 1020-1080℃, the intermediate billet is insulated with a heat preservation cover before entering the hot rolling finishing mill, the finishing mill inlet temperature is 990-1050℃, the final rolling temperature is 820-890℃, after final rolling, laminar flow cooling + air cooling + rapid cooling mode is adopted, the laminar flow cooling rate is greater than 16℃ / s, after cooling to 765-835℃, air cooling is performed, after air cooling for 15-20s, rapid cooling is performed, the rapid cooling rate is greater than 105℃ / s, after rapid cooling to 290-350℃, coiling is performed; after coiling, it is immediately placed in a heated holding pit with a heating temperature of 360-380℃, held for 20-40min, the steel coil is taken out and air cooled to room temperature.

2. The high-formability hot-rolled pickled steel sheet for automobiles according to claim 1, characterized in that, The finished steel plate has a ferrite volume percentage of 8%–12%, a bainite volume percentage of 21%–31%, a martensite volume percentage of 55%–62%, and a retained austenite volume percentage of 2%–5%.

3. The high-formability hot-rolled pickled steel sheet for automobiles according to claim 1, characterized in that, The steel plate has a yield strength ≥ 590 MPa, tensile strength ≥ 890 MPa, longitudinal elongation A ≥ 32%, hole expansion rate ≥ 52%, and longitudinal cold bending 180° D=a is qualified.

4. The high-formability 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 0.75–1.38 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 ≤0.42 g / (m²) over a 168-hour test period. 2 ·h).

5. A method for producing high-formability hot-rolled pickled steel sheet for automobiles as described in any one of claims 1-4, characterized in that, It includes smelting, continuous casting, hot rolling, and pickling processes, specifically: In the hot rolling process: the heating temperature is 1235-1250℃, the holding time is 150-180min, the roughing mill exit temperature is 1020-1080℃, the intermediate billet is insulated with a heat preservation cover before entering the hot rolling finishing mill, the finishing mill inlet temperature is 990-1050℃, the final rolling temperature is 820-890℃, after final rolling, laminar flow cooling + air cooling + rapid cooling mode is adopted, the laminar flow cooling rate is greater than 16℃ / s, after cooling to 765-835℃, air cooling is performed, after air cooling for 15-20s, rapid cooling is performed, the rapid cooling rate is greater than 105℃ / s, after rapid cooling to 290-350℃, coiling is performed; after coiling, it is immediately placed in a heated holding pit with a heating temperature of 360-380℃, held for 20-40min, the steel coil is taken out and air cooled to room temperature.

6. The method for producing high-formability hot-rolled pickled steel sheet for automobiles according to claim 5, characterized in that, In the smelting process: Argon gas is blown for more than 3 minutes when the converter taps out steel, and RH vacuum degassing is maintained for more than 15 minutes.

7. The method for producing high-formability hot-rolled pickled steel sheet for automobiles according to claim 5, characterized in that, In the continuous casting process: the superheat of continuous casting is 15-20℃, 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 280A-380A, the secondary cooling water volume is 0.85-1.15L / kg, light pressure is applied at the end of solidification, and the reduction of the continuous casting billet is 3.0-4.5mm.

8. The method for producing high-formability hot-rolled pickled steel sheet for automobiles according to claim 5, characterized in that, The specifications of the continuously cast billet are (170~235)mm thick × (1040~2000)mm wide. The thickness of the intermediate billet between the roughing and finishing rolling is 37.0~55.0mm and the width is 1040~2000mm.

9. The method for producing high-formability hot-rolled pickled steel sheet for automobiles according to claim 5, characterized in that, The pickling process includes: pre-pickling straightening of the steel plate with an elongation 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 63.0–65℃. The pickling temperature is 8.0℃, the concentration in tank 4 is 145~160g / 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, and the corrosion inhibitor accounts for 0.09%~0.14% of the weight percentage of the pickling solution. The rinsing water temperature is 52~62℃, and the pickling and rinsing speeds are controlled at 80~120m / min. The pickling process is tension pickling with a tension of 40~55kN, and the pickling solution is hydrochloric acid.

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