A 640mpa grade hot-rolled pickled steel sheet for automotive chassis and a method for producing the same

Abstract

The present application belongs to the field of metal material, and particularly relates to a 640MPa-grade hot-rolled pickling steel plate for automobile chassis and a production method thereof. The steel plate of the present application is mainly used for manufacturing automobile chassis structural parts. The chemical components in the steel are C, Si, Mn, Al, Nb, Ti, Ni, Cu, Ta, Zr, Y, 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 of the steel plate is ≥480MPa, the tensile strength is ≥640MPa, the longitudinal elongation A is ≥26%, the hole expansion ratio is 85%-95%, 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, the surface roughness Ra is 1.08-1.48μm, 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 a 640MPa grade hot-rolled pickled steel sheet for automotive chassis 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 CN113755745B discloses a high-expansion hot-rolled pickled steel sheet with a tensile strength of 650MPa. It primarily addresses the technical problem of the mismatch between the stamping and expansion properties of existing 650MPa-grade hot-rolled pickled steel sheets, which cannot meet the manufacturing requirements of complex-shaped automotive parts that balance load-bearing capacity and formability. The steel sheet is mainly used to manufacture automotive chassis structural components with high load-bearing requirements; however, it lacks resistance to alkali corrosion and oxidation, limiting its application environment.

[0004] Chinese patent application CN10484827A discloses a hot-rolled pickled steel sheet with a tensile strength of 600MPa and a low yield strength ratio. This invention primarily addresses the technical problem of mismatched stamping and hole-expanding properties in existing 600MPa-grade hot-rolled pickled steel sheets, which cannot meet the manufacturing requirements of complex-shaped automotive parts. The invention produces a 2.0–4.0mm thick hot-rolled pickled steel sheet with a yield strength ratio ≤0.80 and an elongation after fracture (A0.05). 50mm With a strength of 25-35% and a hole-expanding performance λ≥75%, this steel plate is used for manufacturing automotive structural components. However, it lacks resistance to alkaline corrosion and oxidation, making it unsuitable for manufacturing automotive chassis structural components. Summary of the Invention

[0005] To address the evolving needs of the automotive steel industry, this invention provides a 640MPa grade hot-rolled pickled steel sheet for automotive chassis and its production method. The steel sheet exhibits a yield strength ≥480MPa, tensile strength ≥640MPa, longitudinal elongation A ≥26%, hole expansion rate of 85%–95%, acceptable longitudinal cold bending at 180° (D=a), and a surface roughness Ra of 1.08–1.48μm, along with excellent 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 ≤0.40g / (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 640MPa grade hot-rolled pickled steel sheet for automobile chassis has the following chemical composition by weight percentage: C: 0.063%–0.095%, Si: 0.30%–0.55%, Mn: 1.55%–1.70%, Al: 0.010%–0.040%, Nb: 0.032%–0.045%, Ti: 0.030%–0.055%, Ni: 0.11%–0.15%, Cu: 0.33%–0.45%, Ta: 0.008%–0.013%, Zr: 0.010%–0.015%, 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 this invention consists of 15%–20% ferrite, 75%–85% bainite, and 0%–9% pearlite. The steel plate has a yield strength ≥480 MPa, tensile strength ≥640 MPa, longitudinal elongation A ≥26%, a hole expansion rate of 85%–95%, and passes the longitudinal cold bending test at 180° (D=a). The finished steel plate has a thickness of 1.50–6.00 mm and a surface roughness Ra of 1.08–1.48 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 ≤0.40 g / (m²) over a 168-hour test period. 2 ·h).

[0009] The main function of the composition of a 640MPa grade hot-rolled pickled steel sheet for automobile chassis in this invention is as follows:

[0010] 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 bainite content, thereby improving the formability and hole-expanding properties 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 also benefits the weldability of the steel plate. Therefore, the optimal range for carbon in this invention is 0.063% to 0.095%.

[0011] Silicon (Si): Silicon is one of the key elements in this invention. Sufficient silicon addition 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 and hole-expanding performance from being reduced 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 hot-rolled surface quality, resulting in a large amount of iron oxide scale. Therefore, the silicon content in this invention is 0.30%–0.55%.

[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 expansion 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 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 1.55% to 1.70%.

[0013] P: Phosphorus is an impurity element in steel that readily agglomerates at grain boundaries. High phosphorus content in steel easily leads to the formation of Fe2P particles, reducing the steel's plasticity and toughness. Therefore, a lower phosphorus content is better. 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 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%.

[0016] 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 0.032% and 0.045%.

[0017] Ti: Titanium has significant precipitation strengthening and grain refinement effects. Its effect is mainly achieved through the formation of precipitates with carbon and nitrogen, especially TiN precipitation with nitrogen, which greatly improves 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 enabling the steel plate to achieve excellent mechanical properties and resistance to hydrogen-induced cracking. Higher Ti content also leads to poorer toughness in the weld heat-affected zone; therefore, the Ti addition amount in this invention is 0.030%–0.055%.

[0018] 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 0.11% and 0.15%.

[0019] 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 0.33% and 0.45%.

[0020] 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 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. Therefore, the optimal Ta content range in this invention is between 0.008% and 0.013%.

[0021] 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 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 0.010% to 0.015%.

[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 0.005% to 0.012% in this invention.

[0023] 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%.

[0024] A method for producing 640MPa grade hot-rolled pickled steel sheet for automobile chassis includes smelting, continuous casting, hot rolling, and pickling processes, specifically including:

[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 0.063~0.095%. Argon gas is blown for more than 7 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, the slab is continuously cast. The superheat of continuous casting is 15~20℃ and the continuous casting drawing speed is 1.1~1.6m / 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 200-300A, 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 3.0-4.0mm (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).

[0026] (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 1190-1250℃ 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 a heat insulation cover before entering the hot rolling finishing mill to reduce the temperature drop of the intermediate slab 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 process. High-pressure water descaling is performed before finishing milling. The entry temperature of the finishing mill is 990-1000 mm. The rolling temperature is 1050℃, and the final rolling temperature is 858~908℃. 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 668~725℃, air cooling is performed. After air cooling for 15~20s, rapid cooling is performed with a rapid cooling rate greater than 25℃ / s. After rapid cooling to 498~540℃, coiling is performed. The purpose of laminar flow cooling to 668~725℃ followed by air cooling for 15~20s is to promote the rapid formation of ferrite, which inhibits grain growth while ensuring the ferrite content, thereby refining the ferrite grains. The purpose of rapid cooling to 498~540℃ is to promote the rapid formation of bainite, which inhibits grain growth while ensuring the bainite content, thereby refining the bainite grains. The final microstructure of the rolled steel plate consists of 15%–20% ferrite, 75%–85% bainite, and 0%–9% pearlite.

[0027] (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 1.08–1.48 μm.

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

[0029] 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 of the present invention during use, and obtaining a steel plate with excellent mechanical properties and resistance to hydrogen-induced cracking.

[0030] 2. 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 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.

[0031] 3. 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 plates. 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.

[0032] 4. 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.

[0033] 5. 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, obtaining a fine and uniform grain structure, and improving the strength, toughness, plasticity, and porosity of steel.

[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 microstructure of the steel of this invention consists of ferrite, bainite and pearlite, which significantly improves the mechanical properties and hole expansion performance of the steel plate during the forming process.

[0036] 8. This invention possesses excellent mechanical properties: yield strength ≥ 480 MPa, tensile strength ≥ 640 MPa, longitudinal elongation A ≥ 26%, porosity 85%–95%, longitudinal cold bending at 180° D=a is acceptable, and surface roughness Ra is 1.08–1.48 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 ≤ 0.40 g / (m²) over a 168-hour 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. In the converter smelting, a double-slag method is used to remove phosphorus (P), controlling the P content to ≤0.015%. At the end of the converter smelting, the carbon content is controlled between 0.063% and 0.095%. Argon gas is blown for at least 7 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 1.1-1.6m / min, the electromagnetic stirring current intensity in the secondary cooling zone is controlled at 200A-300A, the secondary cooling water volume is 0.75L / kg-1.00L / 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 3.0-4.0mm, 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 1190–1250℃ for 150–180 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 37.0–55.0 mm and a width of 1040–2000 mm. Before entering the hot finishing mill, the intermediate slab is kept warm. The temperature is maintained by a heat shield 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. The finishing rolling is a 7-stand continuous rolling process. High-pressure water descaling is performed before finishing rolling. The entry temperature of the finishing rolling is 990℃~1050℃, and the final rolling temperature is 858~908℃. After the final rolling, a laminar flow cooling + air cooling + rapid cooling mode is adopted. The laminar flow cooling rate is greater than 16℃ / s. After cooling to 668~725℃, air cooling is performed. After air cooling for 15~20s, rapid cooling is performed with a rapid cooling rate greater than 25℃ / s. After rapid cooling to 498~540℃, the plate is coiled. 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% to 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 to 70 g / L, and the temperature of pickling solution in tank 1 is 77.0 to 86.0℃. The concentration of pickling solution in tank 2 is 72 to 103 g / L, and the temperature of pickling solution in tank 2 is 68.0 to 77.0℃. The concentration of pickling solution in tank 3 is 120 to 140 g / L, and the temperature of pickling solution in tank 3 is 63.0 to 68.0℃. The concentration of pickling solution in tank 4 is 145 to 160 g / L, and the temperature of pickling solution in tank 4 is 55.0 to 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 1.08 to 1.48 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 640MPa grade hot-rolled pickled steel sheet for automobile chassis, characterized in that, The chemical composition of the steel, by weight percentage, is as follows: C: 0.063%–0.074%, Si: 0.30%–0.55%, Mn: 1.55%–1.70%, Al: 0.010%–0.040%, Nb: 0.032%–0.045%, Ti: 0.030%–0.055%, Ni: 0.11%–0.15%, Cu: 0.33%–0.45%, Ta: 0.008%–0.013%, Zr: 0.010%–0.015%, Y: 0.005%–0.012%, with P ≤ 0.015%, S ≤ 0.005%, N ≤ 0.003%, and the balance being Fe and unavoidable impurities. The steel plate has a yield strength ≥480MPa, tensile strength ≥640MPa, longitudinal elongation A ≥26%, hole expansion rate of 85%~95%, and longitudinal cold bending of 180° D=a is qualified. The finished steel plate has a thickness of 1.50–6.00 mm and a surface roughness Ra of 1.08–1.48 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 ≤0.40 g / (m²) over a 168-hour test period. 2 ·h).

2. The 640MPa grade hot-rolled pickled steel sheet for automobile chassis according to claim 1, characterized in that, The microstructure of the finished steel plate consists of 15%–20% ferrite, 75%–85% bainite, and 0%–9% pearlite.

3. A method for producing 640MPa grade hot-rolled pickled steel sheet for automobile chassis 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 described above: the heating temperature is 1190-1250℃, the holding time is 150-180min, the roughing mill exit temperature is 1020-1080℃, the intermediate billet is kept warm by an insulation cover before entering the hot rolling finishing mill, the finishing mill inlet temperature is 990-1050℃, the final rolling temperature is 858-908℃, 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 668-725℃, air cooling is performed, after air cooling for 15-20s, rapid cooling is performed, the rapid cooling rate is greater than 25℃ / s, after rapid cooling to 498-540℃, the billet is coiled.

4. The method for producing 640MPa grade hot-rolled pickled steel sheet for automobile chassis according to claim 3, characterized in that, In the smelting process: argon gas is blown for more than 7 minutes when tapping steel, and RH vacuum degassing is maintained for more than 15 minutes.

5. The method for producing 640MPa grade hot-rolled pickled steel sheet for automobile chassis according to claim 3, characterized in that, In the continuous casting process: the superheat of continuous casting is 15-20℃, the continuous casting billet pulling speed is 1.1-1.6m / min, the electromagnetic stirring current intensity in the secondary cooling zone of the continuous casting stage is controlled at 200-300A, the secondary cooling water volume is 0.75-1.00L / kg, light pressure is applied at the end of solidification, the reduction of the continuous casting billet is 3.0-4.0mm, and the billets are stacked for more than 36 hours after leaving the line.

6. The method for producing 640MPa grade hot-rolled pickled steel sheet for automobile chassis according to claim 3, characterized in that, The specifications of the continuously cast billet are 170-235mm thick × 1040-2000mm wide. The intermediate billet between roughing and finishing rolling has a thickness of 37.0-55.0mm and a width of 1040-2000mm.

7. The method for producing 640MPa grade hot-rolled pickled steel sheet for automobile chassis 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 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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