High-strength pickling automotive structural steel with excellent phosphating properties (370~440MPa grade) and its production method

Abstract

This invention discloses a high-strength pickled automotive structural steel of 370-440 MPa grade with excellent phosphating performance. The chemical composition, by mass percentage, is: C 0.07%-0.1%, Si≤0.03%, Mn 0.7%-1.3%, P≤0.015%, S≤0.005%, Ti≤0.01%, Nb≤0.01%, V≤0.01%, with the remainder being Fe and unavoidable impurities. The product of this invention has a tensile strength of 370-500 MPa, a yield strength of 225-350 MPa, and an elongation ≥34%. It exhibits good phosphating performance, is suitable for novel environmentally friendly phosphating agents, has a crystal size of 3-5 μm, and a phosphating film weight of 1.7-2.3 g / m³. 2 The phosphorus ratio of the phosphate film is ≥0.85. This invention can solve the key technical bottleneck of insufficient phosphate performance of existing 370-440MPa grade automotive pickling structural steel, realizing "acid instead of cold", which not only reduces the procurement cost for users, but also provides a solution for the environmental upgrading of the automotive industry chain.

Description

Technical Field

[0001] This invention belongs to the field of strip steel production technology, and particularly relates to high-strength pickled automotive structural steel of grade 370-440MPa with excellent phosphating properties and its production method. Background Technology

[0002] High-strength automotive structural steel of 370–440 MPa grade is widely used in automotive structures or chassis components. It requires high yield and tensile strength, as well as a certain elongation, to withstand loads during service and to resist deformation during stamping. To improve the corrosion resistance of parts, 370–440 MPa grade automotive structural steel requires phosphating, painting, or powder coating after forming. Phosphating involves forming a water-insoluble phosphate conversion film on the surface, containing numerous tiny pores that allow paint or powder coating to penetrate effectively, significantly enhancing the adhesion between the film and the substrate. Phosphating must be performed in a phosphate solution, and the resulting wastewater poses a certain environmental hazard, requiring further treatment to ensure emissions fall below national standards. In recent years, with the increasing demands for health and environmental protection in society, automobile OEMs or parts manufacturers have adopted environmentally friendly phosphating agents to replace traditional processes. However, the phosphating effect of the new phosphating agents has deteriorated, and the surface quality of pickled steel sheets is slightly worse than that of cold-rolled steel sheets. The new phosphating agents are not well-suited for existing pickled steel sheets, resulting in uneven phosphating film, mottled appearance, and incomplete phosphating crystal coverage.

[0003] Compared to cold-rolled products, pickled steel avoids the cold rolling and annealing processes required for cold rolling, resulting in a shorter production process, lower costs, and effectively reducing procurement costs for users. However, the phosphating performance of existing pickled structural steel is inferior to that of cold-rolled materials, limiting its mass application in the automotive industry. Improving the phosphating performance of automotive pickled structural steel has become an urgent need and development trend in the development of pickled automotive structural steel products.

[0004] CN201610556968.1 discloses a high-surface-quality hot-rolled pickled steel for automobiles with a tensile strength of 370 MPa. Its composition and wt% are: C: 0.050–0.080%, Si≤0.10%, Mn: 0.400–0.700%, P≤0.025%, S≤0.008%, Als: 0.015–0.045%. The production method involves conventional smelting and continuous casting into billets; heating the billets; hot rolling; coiling; laminar flow cooling; and pickling. This invention utilizes the surplus capacity of a cold-rolled pickling production line to produce hot-rolled pickled steel for automobiles. It uses the last stand of a five-stand mill to control the surface roughness of the pickled steel, achieving a surface roughness of 0.8–1.5 μm. This patent controls the surface roughness of the steel strip using a five-stand cold continuous rolling mill, resulting in a complex production process and high manufacturing costs.

[0005] CN201210298855.8 discloses a thin-gauge hot-rolled pickled steel with good formability, the chemical composition of which is as follows (wt%): C: 0.04-0.07%, Si: 0.10-0.30%, Mn: 0.41-0.60%, P≤0.025%, S≤0.012%, Als: 0.010-0.030%; the production steps are: desulfurization of molten iron, converter smelting and alloying treatment; ladle furnace treatment; continuous casting and rolling; high-pressure water descaling; controlled rolling; laminar flow cooling; coiling; pickling; leveling; finishing, and electrostatic oiling. The invention produces hot-rolled pickled steel with a tensile strength ≥370MPa and a thickness ≤1.8mm, which reduces the production difficulty, has good surface quality, and is easy to weld, oil, and paint. However, the subsequent surface treatment of this patented product is painting, which does not require phosphating. Moreover, this patent is produced using a continuous casting and rolling process, which results in poorer surface quality and phosphating performance compared to conventional processes. Summary of the Invention

[0006] Based on the above background, and to overcome the shortcomings of existing technologies, the technical problem to be solved by this invention is a 370-440MPa grade pickled automotive structural steel with excellent phosphating performance and its production method. The product has a tensile strength of 370-500MPa, a yield strength of 225-350MPa, an elongation of ≥34%, good phosphating performance, is suitable for new environmentally friendly phosphating agents, has a crystal size of 3-5μm, and a phosphating film weight of 1.7-2.3g / m³. 2 The phosphorus ratio of the phosphate film is ≥0.85. This technology can solve the key technical bottleneck of insufficient phosphate performance of existing 370-440MPa grade automotive pickling structural steel, realizing "acid instead of cold", which not only reduces the procurement cost for users, but also provides a solution for the environmental upgrading of the automotive industry chain.

[0007] The technical solution adopted by the present invention to solve the above-mentioned problems is as follows:

[0008] A high-strength pickled automotive structural steel of 370-440MPa grade with excellent phosphating properties has the following main chemical composition: Fe, C, Si, Mn, etc., in the following mass percentages: C 0.07%-0.1%, Si≤0.03%, Mn 0.7%-1.3%, P≤0.015%, S≤0.005%, Ti≤0.01%, Nb≤0.01%, V≤0.01%, with the remainder being Fe and unavoidable impurities.

[0009] Preferably, the 370-440MPa grade high-strength pickled automotive structural steel with excellent phosphating performance has the following composition by mass percentage: C 0.075%-0.1%, Si≤0.015%, Mn 1.1%-1.3%, P≤0.012%, S≤0.005%, Ti≤0.01%, Nb≤0.01%, V≤0.01%, with the remainder being Fe and unavoidable impurities. Within this preferred chemical composition range, the product has a tensile strength of 420-500MPa, a yield strength of 275-350MPa, an elongation ≥34%, a crystal size of 3-5μm, and a phosphating film weight of 1.7-2.3g / m². 2 The phosphorus ratio of the phosphating film is ≥0.85.

[0010] This invention also provides a method for producing the above-mentioned high-strength pickled automotive structural steel of 370-440MPa grade with excellent phosphating properties, mainly including continuous casting, billet heating, rough rolling, finish rolling, laminar flow cooling, coiling, pickling, etc., and its main features are:

[0011] (1) Continuous casting: The superheat of the tundish is 10-25℃, and the casting speed is controlled at 0.9-1.3m / min;

[0012] (2) Billet heating: The billet is heated in stages. The furnace entry temperature is ≥600℃, the preheating section temperature is 950~1050℃, and the excess air coefficient is 1.1~1.3; the first heating section temperature is 1090~1150℃, and the excess air coefficient is 1.1~1.3; the second heating section temperature is 1160~1220℃, and the excess air coefficient is 0.9~1.1; the soaking section temperature is 1190~1210℃, and the excess air coefficient is 0.9~1.1; the time for the second heating section and the soaking section is ≤100min, and the total heating time is ≤180min.

[0013] (3) Rough rolling: The billet is rough rolled to a thickness of 30-35mm. During the rough rolling process, the number of descaling times is ≥5 times and the descaling pressure is ≥15MPa.

[0014] (4) Finishing rolling: F1-F7 finishing rolling mills are used, with an initial rolling temperature of 950-1000℃, a reduction rate of F1 and F2 of ≥45%, a final rolling temperature of 830-870℃, and a finishing rolling F7 work roll mileage of 15-30km.

[0015] (5) Laminar flow cooling: Cool to the winding temperature at a rate of 40-70℃ / s;

[0016] (6) Winding: The winding temperature is controlled at 540-580℃;

[0017] (7) Pickling: The elongation rate is 1.0-1.5%, the pickling speed is 60-110m / min, and the total pickling time is 55-75s, to obtain a high-strength pickled automotive structural steel strip with excellent phosphate properties of 370-440MPa.

[0018] Preferably, in the billet heating step, the furnace temperature is 660-730℃, the time for the second heating section plus the soaking section is ≤80min, and the total heating time is ≤160min.

[0019] Preferably, in the finishing rolling step, the initial rolling temperature is 950–980°C and the final rolling temperature is 830–850°C.

[0020] Preferably, the winding temperature is controlled at 540–560°C.

[0021] The alloy types and their contents are selected based on the following reasons:

[0022] C: C can significantly improve the yield strength and tensile strength of steel through solid solution strengthening and phase transformation strengthening, making it one of the most economical elements for increasing strength. However, when the C content is high, the volume fraction of pearlite is high, which is detrimental to phosphating and forming performance. In order to balance mechanical properties, manufacturing costs and phosphating performance, this invention controls the C weight percentage in steel to be 0.07-0.1%.

[0023] Si: Si tends to accumulate on the surface of steel substrates, which prevents the phosphating reaction from taking place and makes it impossible to produce a good phosphating film. In this invention, the weight percentage of Si is controlled to be ≤0.03%.

[0024] Mn: Mn dissolves in ferrite and austenite, which can improve strength, but its strength-improving effect is weaker than that of C and its cost is higher. On the other hand, Mn is prone to dendritic segregation and central segregation during casting. The room temperature structure is prone to forming a two-phase banded structure of ferrite and pearlite, which is not conducive to phosphating performance, formability and uniformity of transverse and longitudinal properties. Taking all factors into consideration, the weight percentage of Mn is controlled at 0.7% to 1.3%.

[0025] P: P is a harmful impurity element in steel. P in steel is prone to segregation, which reduces the toughness and weldability of steel. Therefore, the lower the P content, the better. Taking all factors into consideration, the P content should be ≤0.015%.

[0026] S: Sulfur (S) in steel can worsen performance indicators such as elongation. High S content can easily form sulfide inclusions, making it impossible to form a complete phosphate film in certain areas. At the same time, when the Mn content is low, S can easily cause hot brittleness in steel. Therefore, the S content should be controlled to ≤0.005%.

[0027] Microalloying elements such as Ti, Nb, and V readily combine with N and C elements in the steel to form precipitates. These precipitates form micro-cells with the matrix, leading to enhanced local reactivity. Phosphating films preferentially nucleate and grow rapidly near these micro-cells, resulting in localized phosphating anomalies on the sample surface. Simultaneously, the wettability of the phosphating solution differs between the matrix and heterogeneous particles, preventing the formation of a complete phosphating film in certain areas of the sample surface. Therefore, Ti ≤ 0.01%, Nb ≤ 0.01%, and V ≤ 0.01%.

[0028] The production method of high-strength pickled automotive structural steel of grade 370-440MPa with excellent phosphating performance, as described in this invention, has the following technical characteristics:

[0029] To improve the yield strength and tensile strength of the product, the composition of this invention contains a certain amount of Mn. Mn is prone to dendritic segregation and central segregation during the casting process, which leads to the formation of a two-phase banded structure of ferrite and pearlite at room temperature. This is not conducive to phosphating performance, formability, and uniformity of transverse and longitudinal properties. Lower superheat can improve central segregation, and lowering the casting speed can reduce the proportion of columnar crystals and reduce the degree of dendrite aggregation. However, excessively low superheat and casting speed can easily lead to casting interruption. Therefore, the superheat of the ladle in this invention is controlled at 10-25°C, and the casting speed is controlled at 0.9-1.3 m / min.

[0030] When the billet temperature is ≥1170℃, a Fe2SiO4 / FeO eutectoid liquid phase layer forms and penetrates into the grain boundaries of the matrix. This eutectoid liquid phase layer is difficult to remove by descaling, inhibiting the adhesion and nucleation of phosphide salts on the steel plate surface. This method increases the billet's furnace entry temperature, the temperatures of the preheating section and the first heating section, and extends the time in the preheating and first heating sections to rapidly raise the billet temperature, thereby reducing the billet's residence time in the second heating section and the soaking section. In addition, the heating temperatures of the second heating section and the soaking section are lowered, and a weak oxidizing atmosphere is used to shorten the billet's time in the second heating section and the soaking section, thereby reducing the formation of the Fe2SiO4 / FeO eutectoid liquid phase layer, reducing its penetration depth, making it easier to remove during descaling, and avoiding hindering the phosphating reaction. Because the product of this invention has high strength and is subjected to a large load during the finishing rolling process, in order to avoid scrap steel or roll spalling due to the high load in subsequent rolling processes, and to ensure rolling stability and surface quality, the heating temperature cannot be too low and the heating time cannot be too short. Taking all factors into consideration, this invention controls the furnace entry temperature to be ≥600℃, the preheating section temperature to be 950~1050℃, and the excess air coefficient to be 1.1~1.3; the first heating section temperature to be 1090~1150℃, with an excess air coefficient of 1.1~1.3; the second heating section temperature to be 1160~1220℃, with an excess air coefficient of 0.9~1.1; the soaking section temperature to be 1190~1210℃, with an excess air coefficient of 0.9~1.1; the time for the second heating section and the soaking section to be ≤100min; and the total heating time to be ≤180min. Preferably, the time for the second heating section and the soaking section to be ≤80min; and the total heating time to be ≤160min.

[0031] The reason for controlling the intermediate billet thickness to 30-35 mm in this invention is to reduce the finishing rolling load and control the microstructure of the product after cooling. When the intermediate billet thickness is large, the finishing rolling load is high, easily leading to oxide scale indentation and pitting defects. When the intermediate billet thickness is small, the total reduction rate in finishing rolling is low, resulting in coarse austenite grains and a coarse and uneven microstructure after cooling, which is detrimental to the phosphating and forming properties of the product. Descaling is performed ≥5 times during rough rolling at a pressure ≥15 MPa to reduce the intermediate billet temperature and decrease the formation of the Fe2SiO4 / FeO eutectoid liquid phase layer. Intensive descaling enhances the removal of the Fe2SiO4 / FeO eutectoid liquid phase layer.

[0032] The reason why the initial rolling temperature is controlled at 950-1000℃ and the reduction rate of F1 and F2 is ≥45%, preferably 950-980℃, is to promote the complete recrystallization of austenite through low temperature and high reduction in finishing rolling, thereby obtaining fine austenite grains. The reason why the final rolling temperature is controlled at 830-870℃, preferably 830-850℃, is to form a certain number of dislocations, deformation bands and other defects in the austenite grains, providing nucleation sites for ferrite phase transformation during the cooling process, thereby forming a fine ferrite structure.

[0033] This invention controls the finishing mill F7 work roll mileage to be 15–30 km, and the F1 and F2 reduction rates to be ≥45%. This is because the surface roughness of the steel strip after pickling affects the phosphating effect. If the roughness is too small, the phosphide salt will be difficult to adhere to and nucleate on the steel plate surface; if the roughness is too large, the phosphide salt crystallization will be uneven. This production method controls the finishing mill F7 work roll mileage to be 15–30 km, shifts the rolling load forward, ensures a small rolling load on the downstream stand, and maintains good work roll surface quality, thereby achieving a steel strip surface roughness Ra of 1.2–1.8 μm.

[0034] The composition of this invention contains certain amounts of C and Mn elements, and the balanced microstructure is a two-phase structure of ferrite and pearlite. Since pearlite is difficult to corrode during pickling, while ferrite is easily corroded, the pearlite and ferrite form micro-cells, leading to increased ferrite corrosion depth and hindering the uniformity of the phosphating reaction. Furthermore, pearlite is difficult to dissolve with phosphoric acid, inhibiting phosphide nucleation. This invention controls the laminar cooling rate to 40–70°C / s and the winding temperature to 540–580°C, preferably 540–560°C. Rapid cooling through the ferrite phase transformation region reduces the ferrite grain size, increases the dissolved carbon content in the ferrite, and reduces the pearlite volume fraction.

[0035] Furthermore, during the pickling process, to improve pickling efficiency, a stretching and descaling process is performed before pickling to accelerate the reaction rate between the iron oxide scale and the acid. If the stretching elongation is too low, cracks cannot be formed on the iron oxide scale on the surface of the product of this invention. Conversely, if the stretching elongation is too high, it will reduce the product's elongation. Therefore, the stretching elongation of this invention is 1.0–1.5%. The reason for controlling the pickling speed to 60–110 m / min and the total pickling time to 55–75 s is that the pickling speed and time affect the surface morphology of the substrate. If the pickling speed is too fast and the pickling time is too short, the iron oxide scale will be difficult to remove, delaying the phosphating film formation time. If the pickling speed is too slow and the pickling time is too long, it will lead to an increase in the ferrite corrosion depth, which is not conducive to the uniformity of the phosphating reaction.

[0036] Compared with the prior art, the beneficial effects of the present invention are:

[0037] This invention primarily utilizes carbon and manganese solid solution strengthening to obtain high-strength pickled automotive structural steel of 370–440 MPa grade, dominated by a quasi-polygonal fine-grained ferrite microstructure, avoiding the adverse effects of Nb, V, and Ti microalloying and multiphase microstructure on phosphating. To improve the phosphating performance of the product, the heating temperature, time, and atmosphere of each stage during billet heating are controlled, employing techniques such as sufficient preheating in the preheating stage and reduced oxidation in the postheating stage. Combined with thorough descaling during rough rolling, the formation of the Fe2SiO4 / FeO eutectoid liquid phase layer is reduced, and its removal is enhanced. During finish rolling, low-temperature, high-pressure control ensures sufficient recrystallization of austenite in the upstream stand and generates intragranular defects in austenite in the downstream stand. Combined with laminar rapid cooling and low-temperature coiling, the formation of pearlite microstructure is reduced, resulting in a quasi-polygonal ferrite microstructure. Furthermore, through billet descaling, high-temperature induction heating, finish rolling descaling, finish rolling mileage control, and forward rolling load adjustment, a steel strip surface roughness Ra of 1.2–1.8 μm is achieved.

[0038] The high-strength pickled automotive structural steel strip of 370-440MPa grade obtained by this invention has a uniform surface composition and no local defects, resulting in good phosphating performance. It is suitable for new environmentally friendly phosphating agents, with a crystal size of 3-5μm and a phosphating film weight of 1.7-2.3g / m². 2 The phosphorus ratio of the phosphate film is ≥0.85. This invention enables the "acid-based cooling" process for pickling high-strength automotive structural steel of 370-440MPa grade, providing a solution for users to reduce procurement costs and decrease harmful element emissions from the phosphate process, thus promoting the green transformation and upgrading of the automotive industry chain. Attached Figure Description

[0039] Figure 1 The microstructure of a high-strength pickled automotive structural steel of 370-440 MPa grade with excellent phosphating properties produced using the present invention is shown.

[0040] Figure 2 The microstructure of a high-strength pickled automotive structural steel of grade 370-440MPa with excellent phosphating performance produced by the present invention after phosphating with an environmentally friendly phosphating solution. Detailed Implementation

[0041] To better understand the present invention, the following embodiments further illustrate the content of the invention, but the present invention is not limited to the following embodiments. This section lists 10 sets of embodiments to illustrate the implementation of this process.

[0042] Examples 1-10

[0043] The chemical composition of the 370-440MPa grade high-strength pickled automotive structural steel with excellent phosphating properties provided in Examples 1-10 is shown in Table 1.

[0044] Table 1 Chemical composition (wt%) of each example

[0045] C Si Mn P S Ti Nb V Example 1 0.07 0.005 0.9 0.0119 0.001 0.006 0.008 0.009 Example 2 0.099 0.015 0.95 0.0131 0.001 0.007 0.008 0.01 Example 3 0.075 0.03 1.17 0.016 0.0029 0.007 0.008 0.009 Example 4 0.097 0.006 1.3 0.085 0.0042 0.01 0.007 0.006 Example 5 0.08 0.019 0.79 0.0134 0.0043 0.009 0.006 0.004 Example 6 0.1 0.012 1.25 0.0095 0.0031 0.008 0.005 0.006 Example 7 0.076 0.017 0.7 0.0078 0.0048 0.009 0.007 0.007 Example 8 0.091 0.016 0.86 0.0127 0.0036 0.005 0.005 0.006 Example 9 0.092 0.009 1.22 0.012 0.0032 0.007 0.01 0.008 Example 10 0.084 0.017 1.08 0.0102 0.005 0.006 0.005 0.007

[0046] The production method of the 370-440MPa grade high-strength pickled automotive structural steel with excellent phosphating properties described in Examples 1-10 mainly includes continuous casting, billet heating, rough rolling, finish rolling, laminar flow cooling, coiling, pickling, and other steps. The specific steps are as follows:

[0047] (1) Smelting and casting billets: According to the chemical composition and mass percentage of automotive structural steel in Table 1, the billets are smelted in a converter or electric furnace and refined before continuous casting. The superheat of the tundish is controlled at 10-25℃ and the casting speed is controlled at 0.9-1.3m / min.

[0048] (2) Heating the billet: The billet is heated in sections. The furnace entry temperature is ≥600℃, the preheating section temperature is 950~1050℃, and the excess air coefficient is 1.1~1.3; the first heating section temperature is 1090~1150℃, and the excess air coefficient is 1.1~1.3; the second heating section temperature is 1160~1220℃, and the excess air coefficient is 0.9~1.1; the soaking section temperature is 1190~1210℃, and the excess air coefficient is 0.9~1.1; the time for the second heating section + soaking section is ≤100min, and the total heating time is ≤180min.

[0049] (3) Rough rolling: Rough rolling the billet to a thickness of 30-35mm, descaling ≥5 times during the rough rolling process, and descaling pressure ≥15MPa;

[0050] (4) Finishing rolling: F1-F7 finishing mills are used, with an initial rolling temperature of 950-1000℃, F1 and F2 reduction rates ≥45%, a final rolling temperature of 830-870℃, and a finishing rolling F7 work roll mileage of 15-30km.

[0051] (5) Perform laminar flow cooling: cool to the winding temperature at a rate of 40-70℃ / s;

[0052] (6) Winding: The winding temperature is controlled at 540-580℃;

[0053] (7) Pickling: The elongation rate is 1.0-1.5%, the pickling speed is 60-110m / min, and the total pickling time is 55-75s, to obtain a high-strength pickled automotive structural steel strip with excellent phosphate properties of 370-440MPa.

[0054] The process parameters for each embodiment are shown in Table 2, and the main performance parameters are shown in Table 3.

[0055] Table 2. Continuous casting and heating process parameters for each embodiment.

[0056]

[0057] Table 2 (continued) Continuous casting and heating process parameters for each embodiment

[0058]

[0059] Table 3 Rolling and pickling process parameters for each embodiment

[0060]

[0061]

[0062] Table 3 (continued) Rolling and pickling process parameters for each embodiment

[0063]

[0064] Table 4 Typical mechanical properties and phosphating performance indicators of Examples 1-10

[0065]

[0066] Depend on Figure 1 and Figure 2 It can be seen that the 370-440MPa grade high-strength pickled automotive structural steel products produced by this invention are mainly ferritic with fine and dispersed grains. After phosphating treatment, the phosphating film is complete and the phosphating film crystals are fine and uniform.

[0067] The typical mechanical properties and phosphating performance indicators of the 370-440MPa grade high-strength pickled automotive structural steel products produced in Examples 1-10 are shown in Table 4. The tensile strength of the products is 370-500MPa, the yield strength is 225-350MPa, and the elongation is ≥34%. After phosphating with a new type of environmentally friendly phosphating agent, the crystal size is 3-5μm, and the weight of the phosphating film is 1.7-2.3g / m². 2 The phosphating film has a P ratio ≥ 0.85, exhibiting excellent phosphating performance; moreover, the surface composition is uniform and free of local defects. Examples 4, 6, and 9 utilize a more optimized composition (C 0.075–0.1%, Si ≤ 0.015%, Mn 1.1–1.3%, P ≤ 0.012%) to achieve higher mechanical properties, with tensile strength of 420–500 MPa, yield strength of 275–350 MPa, and elongation ≥ 34%. The mechanical properties were tested according to GB / T 228.1, and the phosphating film was tested according to GB 38933.

[0068] The above description is only a preferred embodiment of the present invention. It should be noted that those skilled in the art can make several improvements and modifications without departing from the inventive concept of the present invention, and these all fall within the protection scope of the present invention.

Claims

1. A high-strength pickling automotive structural steel with excellent phosphating properties, graded 370~440MPa, characterized in that: The chemical composition by mass percentage is as follows: C 0.07%~0.1%, Si 0.005%~0.019%, Mn 0.7%~0.95%, P≤0.015%, S≤0.005%, Ti≤0.009%, Nb≤0.008%, V≤0.009%, with the remainder being Fe and unavoidable impurities, and Ti, Nb and V are not all zero; The production method of the 370~440MPa grade high-strength pickled automotive structural steel with excellent phosphating properties includes the following steps: (1) Continuous casting: The superheat of the tundish is 10~25℃, and the casting speed is controlled at 0.9~1.3m / min; (2) Billet heating: The billet is heated in sections. The furnace entry temperature is ≥660℃, the preheating section temperature is 950~1050℃, and the excess air coefficient is 1.1~1.3; the first heating section temperature is 1090~1150℃, and the excess air coefficient is 1.1~1.3; the second heating section temperature is 1160~1220℃, and the excess air coefficient is 0.9~1.1; the soaking section temperature is 1190~1210℃, and the excess air coefficient is 0.9~1.1; the time for the second heating section and the soaking section is ≤100min, and the total heating time is ≤180min. (3) Rough rolling: The billet is rough rolled to a thickness of 30~35mm. During the rough rolling process, the number of descaling times is ≥5 times and the descaling pressure is ≥15MPa; (4) Finishing rolling: F1-F7 finishing rolling mills are used, with an initial rolling temperature of 950~1000℃, a reduction rate of F1 and F2 of ≥45%, a final rolling temperature of 830~870℃, and a finishing rolling F7 work roll mileage of 15~30km; (5) Laminar flow cooling: Cool to the winding temperature at a rate of 40~70℃ / s; (6) Winding: The winding temperature is controlled at 540~577℃; (7) Pickling: The tensile elongation is 1.0~1.5%, the pickling speed is 60~110m / min, and the total pickling time is 55~75s. High-strength pickled automotive structural steel with excellent phosphate properties of 370~440MPa is obtained. Its tensile strength is 370~500MPa, yield strength is 225~350MPa, product elongation is ≥34%, phosphate crystal size is 3~5μm, and phosphate film weight is 1.7~2.3g / m 2 The phosphorus ratio of the phosphating film is ≥0.

85.

2. The production method of a high-strength pickled automotive structural steel with excellent phosphating properties (370-440 MPa grade) as described in claim 1, comprising the steps of continuous casting, billet heating, rough rolling, finish rolling, laminar flow cooling, coiling, and pickling, characterized in that: (1) Continuous casting: The superheat of the tundish is 10~25℃, and the casting speed is controlled at 0.9~1.3m / min; (2) Billet heating: The billet is heated in sections. The furnace entry temperature is ≥660℃, the preheating section temperature is 950~1050℃, and the excess air coefficient is 1.1~1.3; the first heating section temperature is 1090~1150℃, and the excess air coefficient is 1.1~1.3; the second heating section temperature is 1160~1220℃, and the excess air coefficient is 0.9~1.1; the soaking section temperature is 1190~1210℃, and the excess air coefficient is 0.9~1.1; the time for the second heating section and the soaking section is ≤100min, and the total heating time is ≤180min. (3) Rough rolling: The billet is rough rolled to a thickness of 30~35mm. During the rough rolling process, the number of descaling times is ≥5 times and the descaling pressure is ≥15MPa; (4) Finishing rolling: F1-F7 finishing rolling mills are used, with an initial rolling temperature of 950~1000℃, a reduction rate of F1 and F2 of ≥45%, a final rolling temperature of 830~870℃, and a finishing rolling F7 work roll mileage of 15~30km; (5) Laminar flow cooling: Cool to the winding temperature at a rate of 40~70℃ / s; (6) Winding: The winding temperature is controlled at 540~577℃; (7) Pickling: The elongation rate is 1.0~1.5%, the pickling speed is 60~110m / min, and the total pickling time is 55~75s, so as to obtain high-strength pickled automotive structural steel with excellent phosphate properties of 370~440MPa.

3. The method for producing high-strength pickled automotive structural steel with excellent phosphating properties (370-440 MPa grade) according to claim 2, characterized in that: In the billet heating process, the furnace temperature is 660~730℃, the time for the second heating section and the soaking section is ≤80min, and the total heating time is ≤160min.

4. The method for producing high-strength pickled automotive structural steel with excellent phosphating properties (370-440 MPa grade) according to claim 2, characterized in that: In the finishing rolling process, the initial rolling temperature is 950~980℃, and the final rolling temperature is 830~850℃.

5. The method for producing high-strength pickled automotive structural steel of 370~440MPa grade with excellent phosphating properties according to claim 2, characterized in that: During the winding process, the winding temperature is controlled at 540~560℃.

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