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

Abstract

This invention belongs to the field of metallic materials, and specifically relates to a high-strength 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. The chemical composition of the steel is designed as follows: C, Si, Mn, Al, Nb, Ti, Ni, Cu, Cr, Mo, Ta, Pt, Y, with P ≤ 0.015%, S ≤ 0.005%, N ≤ 0.003%, and the balance being Fe and unavoidable impurities. This invention possesses excellent mechanical properties: the steel sheet has a yield strength ≥ 660 MPa, tensile strength ≥ 820 MPa, transverse elongation A ≥ 22%, and a qualified transverse cold bending rate of 180° D=a; the surface roughness Ra of the steel sheet is 0.75~1.20 μm; and the corrosion rate is ≤ 0.58 g / (m²). 2 ·h).

Description

Technical Field

[0001] This invention belongs to the field of metallic materials, and specifically relates to a high-strength 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 CN119491158A discloses a hot-rolled pickled steel plate for saw blades and its manufacturing method. This invention addresses the technical problems of existing 2.5-7.0mm thick high-carbon, high-manganese steel for saw blades, which suffers from segregation, poor microstructure uniformity, and coarse pearlite structure. The technical solution is a hot-rolled pickled steel plate for saw blades with the following chemical composition by weight percentage: C: 0.75%-0.85%, Si: 0.20%-0.50%, Mn: 0.20%-0.50%, P≤0.020%, S≤0.005%, Al: 0.01%-0.02%, N≤0.0060%, Cr: 0.4%-0.8%, V: 0.18%-0.28%, with the balance being Fe and unavoidable impurity elements. The metallographic structure of the hot-rolled pickled steel sheet of this invention is ultrafine pearlite with a lamellar spacing of 0.4–0.8 μm. This steel sheet is used in the production of circular saw blades, chainsaws, frame saws, etc. However, it does not meet the requirements for use in automotive chassis structural components, nor does it possess sufficient corrosion resistance and oxidation resistance.

[0004] Chinese patent application CN119491161A discloses a hot-rolled high weathering steel plate with an extremely low yield strength ratio and a tensile strength of 850 MPa, its manufacturing method, and its application. The chemical composition of the steel plate, by weight percentage, is as follows: C: 0.06%–0.08%, Si: 0.40%–0.60%, Mn: 0.40%–0.60%, P: ≤0.015%, S: ≤0.005%, Cr: 2.50%–3.00%, Cu: 0.25%–0.30%, Ti: 0.13%–0.15%, Alt: 0.020%–0.040%, N ≤0.0040%, O ≤0.0030%, with the balance being Fe and unavoidable inclusions. This invention employs a high-Cr + high-Ti design to achieve a tensile strength of 850MPa and a weather resistance index (I) ≥8.0. It eliminates the addition of precious metal Ni, and the amount of Cr, a strong hardenability element, exceeds 2.50% in the steel. Combined with controlled rolling and cooling processes, it achieves a fine-grained ferrite + bainite dual-phase microstructure, while fully utilizing the precipitation strengthening and grain refinement strengthening effects of Ti. This results in high strength and an extremely low yield strength ratio (<0.65), excellent formability, and a corrosion rate of <40% compared to Q345B, exhibiting good atmospheric corrosion resistance. However, it lacks resistance to alkali solutions and is not suitable for producing high-strength automotive chassis structural components. Summary of the Invention

[0005] To address the development needs of the automotive steel industry, this invention provides a high-strength hot-rolled pickled steel sheet for automobiles and its production method. The steel sheet has a yield strength ≥660MPa, tensile strength ≥820MPa, transverse elongation A ≥22%, and meets the acceptable transverse cold bending standard (D=a) at 180°. It also has a surface roughness Ra of 0.75~1.20μm and exhibits 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 ≤0.58g / (m²) over a 144h test period. 2 ·h).

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

[0007] A high-strength hot-rolled pickled steel sheet for automobiles, characterized in that the chemical composition of the steel, by weight percentage, is as follows: C: 0.072%–0.140%, Si: 0.43%–0.66%, Mn: 1.66%–1.83%, Al: 0.010%–0.040%, Nb: 0.030%–0.055%, Ti: 0.065%–0.095%, Ni: 0–0.10%, Cu: 0–0.05%, Cr: 0.20%–0.80%, Mo: 0–0.15%, Ta: 0.010%–0.025%, Pt: 0.002%–0.007%, Y: 0.005%–0.012%, with P ≤ 0.015%, S ≤ 0.005%, N ≤ 0.003%, and the balance being Fe and unavoidable impurities.

[0008] The microstructure of the finished steel plate of the present invention is 10% to 15% ferrite, 75% to 85% bainite, 5% to 10% martensite, and 0% to 3% retained austenite.

[0009] The steel plate of this invention has a yield strength ≥660MPa, tensile strength ≥820MPa, transverse elongation A ≥22%, and is qualified for transverse cold bending of 180° D=a (where a represents the nominal thickness of the tested sample). The finished steel plate has a thickness of 1.50~6.00mm and a surface roughness Ra of 0.75~1.20μm. 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 ≤0.58g / (m²) over a 144h test period. 2 ·h).

[0010] The main function of the composition of a high-strength hot-rolled pickled steel sheet for automobiles in this invention is as follows:

[0011] C: Carbon is a common strengthening element in steel. Interstitial carbon atoms cause certain lattice distortions in the matrix, playing a role in solid solution strengthening. In this invention, carbon ensures the content of bainite and martensite, thereby improving the formability of the steel plate. Too low a carbon content will not yield the mechanical properties of the steel plate described in this invention, while too high a content will cause the steel plate to 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 for carbon in this invention is 0.072% to 0.140%.

[0012] Silicon (Si): Silicon is one of the key elements in this invention. Sufficient silicon content ensures the strength of the ferrite matrix. Furthermore, adequate silicon content reduces inclusions in the steel, inhibits their formation, and prevents the steel plate's mechanical properties from being compromised by inclusions. 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 0.43%–0.66%.

[0013] 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 undercooled austenite is unstable, reducing the plasticity and toughness of the steel plate. Furthermore, the added manganese content should not exceed the range 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 and inhibit bainite formation, which is also not conducive to the comprehensive performance of the steel plate. In addition, excessive manganese content will lead to poor weldability of the steel plate. Therefore, considering all factors, this invention selects a manganese content of 1.66% to 1.83%.

[0014] 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%.

[0015] 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%.

[0016] Al: In traditional steelmaking processes, Al is a deoxidizer. Al can also combine with nitrogen in steel to form AlN, refining the grain size. Simultaneously, together with Si, it inhibits cementite precipitation, increases the austenitizing temperature, facilitates better selection of the process window, and accelerates bainite transformation. Excessive Al content will cause nozzle blockage during continuous casting, affecting production efficiency and increasing production costs. Therefore, in this invention, the Al content is limited to 0.010%–0.040%.

[0017] 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 degrade the toughness of the steel plate, Nb combines with impurity N elements in the steel to form NbN, thus achieving 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; however, 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 achieve excellent mechanical properties, the optimal range of Nb content in this invention is between 0.030% and 0.055%.

[0018] Ti: Titanium can effectively delay the recrystallization of deformed austenite, prevent austenite grain growth, increase the recrystallization temperature of austenite, refine grains, and improve the strength and toughness of steel. Moreover, Ti is a strong carbide and nitride forming element, which can combine with carbon and nitrogen to form stable and fine carbide and nitride, playing a significant precipitation strengthening role. Therefore, the titanium content in this invention is selected from 0.065% to 0.095%.

[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% and 0.10%.

[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% and 0.05%.

[0021] 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 optimal range of Cr content in this invention is between 0.20% and 0.80%.

[0022] Mo: Molybdenum is a carbide-forming element that can improve the strength and toughness of steel plates. Mo can significantly improve the stability of austenite, increase the hardenability of steel, and is conducive to the formation of martensite. Therefore, the Mo content in this invention is selected to be 0 to 0.15%.

[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. 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 0.010% and 0.025%.

[0024] Pt: Platinum itself is a chemically inert metal with excellent chemical stability and corrosion resistance. In steel, platinum reacts with oxygen to form a dense oxide protective film. This protective film isolates the steel from direct contact with corrosive media, thereby slowing down the corrosion process. Furthermore, it exhibits good corrosion resistance to most alkaline solutions (including strong alkalis) at room temperature and is not prone to chemical reactions. The addition of platinum can alter the electrochemical properties of steel, potentially assisting or enhancing the formation and stability of the passivation film on the steel surface. It can reduce the electrochemical corrosion activity of steel in alkaline media, improve the corrosion resistance and oxidation resistance of steel, and extend the service life of steel in highly corrosive environments and at high temperatures. Therefore, this invention limits the Pt content to 0.002%–0.007%.

[0025] Y: Yttrium can refine the grains in steel, enhance the strength and plasticity of grain boundaries, which is beneficial to the improvement of mechanical properties. It can 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 0.005% to 0.012% 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-strength 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 0.072% to 0.140%. Argon gas is blown for more than 6 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 18 minutes. Calcium treatment is carried out in the refining LF process. After that, slab continuous casting is carried out. The superheating temperature of continuous casting is 20 to 25℃, and the continuous casting drawing speed is 0.9 to 1.4 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 250-350A, and the secondary cooling water volume to 0.75-1.00L / 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, apply light pressure, and reduce the billet by 3.0-5.0mm (to reduce the central porosity and segregation of the billet). Stack the billets after casting for more than 36 hours (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 (155-245) mm and a width of (1040-2000) mm is loaded into a walking beam furnace for heating at a temperature of 1210-1250℃ 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 ≥1000℃. The thickness of the intermediate slab is 37.0-55.0 mm and the width is 1040-2000 mm. The intermediate slab is then fed into the hot finishing mill. The unit is insulated with a heat preservation cover to reduce the temperature drop of the intermediate billet on the delay roller table and the temperature difference in the head, tail and width directions. The finishing mill is a 7-stand continuous rolling mill. High-pressure water descaling is performed before finishing milling. The entry temperature of the finishing mill is ≤1050℃ and the final rolling temperature is 862~922℃. After final rolling, laminar flow cooling + air cooling + rapid cooling mode is adopted. The laminar flow cooling rate is greater than 18℃ / s. After cooling to 572~625℃, air cooling is performed. After air cooling for 10~15s, rapid cooling is performed. The rapid cooling rate is greater than 27℃ / s. After rapid cooling to 355~408℃, the billet is coiled. After laminar cooling to 572–625℃, air cooling for 10–15 seconds is performed to rapidly generate bainite, inhibiting grain growth while ensuring a sufficient bainite content, thus refining the bainite grains. Rapid cooling to 355–408℃ further promotes rapid martensite formation, inhibiting grain growth while ensuring a sufficient martensite content, thereby refining the martensite grains. The final microstructure of the rolled steel plate consists of 10%–15% ferrite, 75%–85% bainite, 5%–10% martensite, and 0%–3% retained austenite.

[0030] (3) The pickling process includes: continuously cleaning the iron oxide scale of the steel plate with hydrochloric acid, uncoiling the hot-rolled coil for pickling on the pickling line, leveling the steel plate before pickling, with a leveling elongation of 0.7% to 1.2%, using hydrochloric acid as the pickling solution, and the pickling tank is divided into 4 tanks. The concentration of the pickling solution in tank 1 is 45 to 65 g / L, and the temperature of the pickling solution in tank 1 is 78.0 to 88.0℃. The concentration of the pickling solution in tank 2 is 75 to 105 g / L, and the temperature of the pickling solution in tank 2 is 70.0 to 78.0℃. The concentration of the pickling solution in tank 3 is 125 to 145 g / L, and the temperature of the pickling solution in tank 3 is 65.0 to 105 g / L. The pickling temperature is 70.0℃, the concentration in tank 4 is 155~170g / L, and the temperature of the pickling solution in tank 4 is 55.0~65.0℃. 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 50~60℃, and the pickling and rinsing speeds are controlled at 80~120m / min. The pickling process is under tension, with a tension of 60~75KN. Finally, the product is coated with oil and rolled up to obtain the finished product, which has a thickness of 1.50~6.00mm and a surface roughness Ra of 0.75~1.20μm.

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

[0032] 1. 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 block 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.

[0033] 2. Copper plays a role in solid solution strengthening and precipitation strengthening. Furthermore, Cu reduces work hardening, improves the plasticity of steel plates, and in alkaline environments, it forms a protective film on the steel surface containing copper oxide (CuO) or basic copper salts. 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.

[0034] 3. 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, improving the stability of residual austenite after coiling, and ensuring that a certain amount of residual austenite exists 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, especially in strongly alkaline (such as sodium hydroxide) and high-temperature alkaline environments.

[0035] 4. Molybdenum can improve the strength and toughness of steel plates, significantly improve the stability of austenite, increase the hardenability of steel, and facilitate the formation of martensite.

[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. 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. Platinum possesses excellent chemical stability and corrosion resistance. In steel, platinum reacts with oxygen to form a dense oxide protective film. This film isolates the steel from direct contact with corrosive media, thus slowing down the corrosion process. Furthermore, it exhibits good corrosion resistance to most alkaline solutions (including strong alkalis) at room temperature and is not prone to chemical reactions. The addition of platinum can alter the electrochemical properties of steel, potentially aiding or enhancing the formation and stability of the passivation film on the steel surface. It can reduce the electrochemical corrosion activity of steel in alkaline media, improve its corrosion resistance and oxidation resistance, and extend its service life in highly corrosive environments and at high temperatures.

[0038] 7. The addition of Y can refine the grains in steel, enhance the strength and plasticity of grain boundaries, which is beneficial to the improvement of mechanical properties. It can also improve the weldability and oxidation resistance of steel, thereby increasing the service life of steel at high temperatures.

[0039] 8. The microstructure of the steel of this invention consists of ferrite, bainite, martensite and retained austenite, which significantly improves the comprehensive mechanical properties of the steel plate during the forming process.

[0040] 9. This invention possesses excellent mechanical properties: yield strength ≥ 660 MPa, tensile strength ≥ 820 MPa, transverse elongation A ≥ 22%, transverse cold bending at 180° D = a (where a represents the nominal thickness of the tested sample), and surface roughness Ra of 0.75–1.20 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 is ≤ 0.58 g / (m²) over a 144-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 0.072% and 0.140%. Argon gas is blown for at least 6 minutes during tapping, followed by the LF+RH process, with strict control of H and O contents: H ≤0.0002%, O ≤0.0015%. RH vacuum degassing is maintained for 18 minutes. The above process involves calcium treatment in the refining LF process, followed by slab continuous casting. The superheating temperature during continuous casting is 20–25°C, the casting speed is 0.9–1.4 m / min, the electromagnetic stirring current intensity in the secondary cooling zone during continuous casting is controlled at 250–350 A, the secondary cooling water volume is 0.75 L / kg–1.00 L / 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 3.0–5.0 mm, and the slabs are stacked for more than 36 hours after being removed from the line. A continuously cast slab (155-245 mm thick × 1040-2000 mm wide) is loaded into a walking beam furnace for heating at 1210-1250℃ for 150-180 min. Roughing is performed using a 3+3 rolling process (R1 and R2 are rolled in 3 passes), for a total of 6 passes. The roughing mill exit temperature is ≥1000℃. The intermediate slab thickness is 37.0-55.0 mm and the width is 1040-2000 mm. Before entering the hot finishing mill, the intermediate slab is insulated with a heat shield to reduce temperature drop on the delay roller table and temperature differences at the head, tail, and width directions. Finishing is a 7-stand continuous rolling process. High-pressure water descaling is performed before finishing. The finishing mill inlet temperature is ≤1050℃, and the final rolling temperature is 862-922℃. After finishing, laminar flow cooling + air cooling + rapid cooling is used. The rate of cooling is greater than 18℃ / s. After cooling to 572-625℃, air cooling is performed. After air cooling for 10-15 seconds, rapid cooling is performed with a rate greater than 27℃ / s. After rapid cooling to 355-408℃, the coil is then coiled. After hot rolling, the coil is uncoiled on the pickling line and pickled with hydrochloric acid. Before pickling, the steel plate is tension-sharpened with an elongation of 0.7%-1.2%. The pickling solution is hydrochloric acid, and the pickling tank is divided into 4 tanks, with tank number 1 being concentrated hydrochloric acid. The concentration of pickling solution in tank 1 is 45–65 g / L, the temperature of pickling solution in tank 2 is 78.0–88.0℃, the concentration of pickling solution in tank 2 is 75–105 g / L, the temperature of pickling solution in tank 2 is 70.0–78.0℃, the concentration of pickling solution in tank 3 is 125–145 g / L, the temperature of pickling solution in tank 3 is 65.0–70.0℃, and the concentration of pickling solution in tank 4 is 155–170 g / L, the temperature of pickling solution in tank 4 is 55.0–65.0℃.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 50 to 60°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 60 to 75 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 1.20 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-strength hot-rolled pickled steel sheet for automobiles, characterized in that, The chemical composition of the steel, by weight percentage, is as follows: C: 0.072%–0.140%, Si: 0.43%–0.66%, Mn: 1.66%–1.83%, Al: 0.010%–0.040%, Nb: 0.030%–0.055%, Ti: 0.065%–0.095%, Ni: 0.05%–0.10%, Cu: 0.015%–0.05%, Cr: 0.72%–0.80%, Mo: 0.11%–0.15%, Ta: 0.010%–0.025%, Pt: 0.003%–0.007%, Y: 0.005%–0.012%, with P ≤ 0.015%, S ≤ 0.005%, N ≤ 0.003%, and the balance being Fe and unavoidable impurities. The production method of high-strength hot-rolled pickled steel sheet for automobiles includes smelting, continuous casting, hot rolling, and pickling processes, specifically: In the hot rolling process: the heating temperature is 1210~1250℃, the holding time is 150~180min, the roughing mill exit temperature is ≥1000℃, the intermediate billet is kept warm by an insulation cover before entering the hot rolling finishing mill, the finishing mill inlet temperature is ≤1050℃, the final rolling temperature is 862~922℃, after final rolling, laminar flow cooling + air cooling + rapid cooling mode is adopted, the laminar flow cooling rate is greater than 18℃ / s, after cooling to 572~625℃, air cooling is performed, after air cooling for 10~15s, rapid cooling is performed, the rapid cooling rate is greater than 27℃ / s, after rapid cooling to 355~408℃, the billet is coiled. The finished steel plate has a thickness of 1.50–6.00 mm and a surface roughness Ra of 0.75–1.20 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 ≤0.58 g / (m²) over a 144-hour test period. 2 ·h); The microstructure of the finished steel plate is 10%–15% ferrite, 75%–85% bainite, 5%–10% martensite, and 0%–3% retained austenite.

2. The high-strength hot-rolled pickled steel sheet for automobiles according to claim 1, characterized in that, The steel plate has a yield strength ≥660MPa, tensile strength ≥820MPa, transverse elongation A ≥22%, and transverse cold bending at 180° D=a is qualified.

3. A method for producing high-strength hot-rolled pickled steel sheet for automobiles as described in claim 1 or 2, characterized in that, It includes smelting, continuous casting, hot rolling, and pickling processes, specifically: In the hot rolling process: the heating temperature is 1210~1250℃, the holding time is 150~180min, the roughing mill exit temperature is ≥1000℃, the intermediate billet is kept warm by an insulation cover before entering the hot rolling finishing mill, the finishing mill inlet temperature is ≤1050℃, the final rolling temperature is 862~922℃, after final rolling, laminar flow cooling + air cooling + rapid cooling mode is adopted, the laminar flow cooling rate is greater than 18℃ / s, after cooling to 572~625℃, air cooling is carried out, after air cooling for 10~15s, rapid cooling is carried out, the rapid cooling rate is greater than 27℃ / s, after rapid cooling to 355~408℃, coiling is carried out.

4. The method for producing high-strength hot-rolled pickled steel sheet for automobiles according to claim 3, characterized in that, In the smelting process: argon gas is blown for more than 6 minutes when the converter taps out steel, and RH vacuum degassing is maintained for more than 18 minutes.

5. The method for producing high-strength hot-rolled pickled steel sheet for automobiles according to claim 3, characterized in that, In the continuous casting process: the superheat of continuous casting is 20-25℃, the continuous casting billet pulling speed is 0.9-1.4m / min, the electromagnetic stirring current intensity in the secondary cooling zone of the continuous casting stage is controlled at 250-350A, the secondary cooling water volume is 0.75-1.00L / kg, a light pressure is applied in the horizontal section of the secondary cooling zone, that is, at the end of solidification, the reduction of the continuous casting billet is 3.0-5.0mm, and the billets are stacked for more than 36 hours after leaving the line.

6. The method for producing high-strength hot-rolled pickled steel sheet for automobiles according to claim 3, characterized in that, The specifications of the continuously cast billet are (155~245)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.

7. The method for producing high-strength hot-rolled pickled steel sheet for automobiles according to claim 3, 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 45–65 g / L and a pickling solution temperature of 78.0–88.0℃; Tank 2 has a concentration of 75–105 g / L and a pickling solution temperature of 70.0–78.0℃; Tank 3 has a concentration of 125–145 g / L and a pickling solution temperature of 65.0–78.0℃. The concentration in tank 4 is 155–170 g / L, and the pickling solution temperature in tank 4 is 55.0–65.0℃. During pickling, a corrosion inhibitor is added to the acid solution, and the corrosion inhibitor accounts for 0.09%–0.14% of the weight of the pickling solution. The rinsing water temperature is 50–60℃, and the pickling and rinsing speeds are controlled at 80–120 m / min. The pickling process is under tension, with a tension of 60–75 kN. The pickling solution is hydrochloric acid.

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