High-plasticity hot-rolled pickled steel plate with thickness of more than 5.1 mm for vehicle coil spring

Abstract

The invention discloses a high-plasticity hot-rolled pickled steel plate with the thickness of more than 5.1 mm for a vehicle coil spring. The high-plasticity hot-rolled pickled steel plate mainly solves the technical problems that an existing hot-rolled pickled steel plate with the thickness of 5.1-8.5 mm and the width of 1000-1300mm for the vehicle coil spring is low in plasticity, difficult in pickling and difficult in surface scale removal. According to the technical scheme, the high-plasticity hot-rolled pickled steel plate for the vehicle coil spring comprises 0.56%-0.63% of C, 1.6%-2.0% of Si, 0.7%-1.0% of Mn, less than or equal to 0.020% of P, less than or equal to 0.005% of S, less than or equal to 0.01% of Al, less than or equal to 0.004% of N, 0.1%-0.2% of Cr and the balance Fe and inevitable impurity elements. The percentage elongation after fracture # imgabs0 # of the hot-rolled pickled steel plate with the thickness of 5.1-8.5 mm and the width of 1000-1300mm is 18%-25%, and the steel plate is qualified when d is equal to 2a in a 25-degree bending test.

Description

Technical Field

[0001] The present invention relates to a spring steel, in particular to a high-plasticity hot-rolled pickled steel plate with a thickness of 5.1 mm or more for vehicle coil springs and a manufacturing method thereof; specifically, it relates to a high-plasticity hot-rolled pickled steel plate with a thickness of 5.1-8.5 mm and a width of 1000-1300 mm for vehicle coil springs produced by continuous casting slabs and hot rolling, and belongs to the technical field of iron-based alloys. Background Art

[0002] Silicon manganese spring steel is one of the most widely used spring steel varieties. After heat treatment, it has good elasticity and plasticity and can withstand large deformation without breaking. Therefore, the product is widely used in spiral springs and flat springs. However, like most high-carbon tool steels, due to the high carbon content, problems such as organizational segregation, intergranular oxidation, and surface decarburization will inevitably occur during the production process. Usually, as the width and thickness of the steel plate increase, the above problems become more obvious, and it cannot meet the service life requirements of leaf springs and coil springs for large vehicles that need to withstand high load stress. Therefore, this product can only be produced from small billets with a width of less than 600mm, which brings certain restrictions to the application of this type of product.

[0003] Recently, with the advancement of technology, some companies have been able to produce wide plate steel coils with a width of about 600mm.

[0004] The Chinese patent application with application publication number CN114807555A discloses a method for producing a high carbon and high silicon spring steel hot rolled strip, the steel plate thickness is 2.5-14mm, the width is 800-1650mm, the method adopts 850-920℃ finishing rolling temperature, sparse cooling laminar cooling method, 680-720℃ coiling temperature, and can produce a yield strength of 500-650MPa and a tensile strength R m 900~1030MPa, elongation after fracture A 50 The elongation of the hot-rolled steel plate is greater than 8%; in the disclosed embodiment, the elongation of the 4.0-14.0 mm steel plate is 10-18%; the elongation of the steel plate is low.

[0005] For hot-rolled steel plates, as the thickness of the steel plate increases, in order to reduce the tension of the coiler, the coiling temperature usually needs to be increased by 10-20°C, but at the same time, as the thickness increases, the temperature difference between the core and the surface of the steel plate also increases. The actual cooling rate of the steel plate after hot rolling is quite different from that of thin materials, which is accompanied by the time change of the austenite-pearlite phase transformation, thus affecting the final product structure morphology and product performance. In order to ensure the stability of performance, the thick plate actually needs to lower the coiling temperature. In actual production, the temperature difference between the surface and the core of thin products below 5mm is small, and the effect of thickness on the actual temperature drop can generally be ignored. However, for thick products exceeding 5mm, the coiling process needs to be reasonably set according to the balance between the above two. In terms of steel plate structure, the higher the coiling temperature, the coarser the final structure grains, the lower the strip strength, and the higher the plasticity.

[0006] In addition, for high-alloy spring steels, due to the extremely high Si content, hot-rolled steel sheets usually have low plasticity. When pickling and uncoiling, especially when thick and wide plates are uncoiling, the hot-rolled coils need to withstand certain reverse bending, and the risk of fracture is extremely high, which has always been a technical problem for pickling thick plates of this type of steel. At the same time, during the hot rolling process, iron silicate pinning will inevitably form at the interface between the steel base and the iron oxide scale, greatly increasing the difficulty of removing the iron oxide scale. In the slow cooling process after hot rolling, the high corrosion-resistant alloy oxide formed by Si at the interface between the steel base and the iron oxide scale further increases the difficulty of removing the iron oxide scale of this type of steel.

[0007] Therefore, the existing technology cannot produce wide and thick plate pickled spring steel with high plasticity and excellent organizational properties. Summary of the invention

[0008] The purpose of the present invention is to provide a high-plasticity hot-rolled pickled steel plate with a thickness of 5.1 mm or more for vehicle coil springs and a manufacturing method thereof, mainly to solve the technical problems of low plasticity, difficulty in pickling and difficulty in removing surface oxide scale of the existing hot-rolled pickled steel plate with a thickness of 5.1 to 8.5 mm and a width of 1000 to 1300 mm for vehicle coil springs.

[0009] The technical idea of ​​the present invention is to achieve the uniformity of the structure, strength and plasticity requirements of hot-rolled pickled steel plates for vehicle coil springs with a thickness of 5.1 to 8.5 mm and a width of 1000 to 1300 mm through process designs such as hot rolling heating process, rolling process and slow cooling process; in the pickling process, through the reasonable setting of the pickling process, the efficient removal of the oxide scale of thick-gauge hot-rolled high-Si steel can be achieved.

[0010] The technical solution adopted by the present invention is a high-plasticity hot-rolled pickled steel plate with a thickness of more than 5.1 mm for vehicle coil springs, whose chemical composition weight percentage is: C: 0.56% to 0.63%, Si: 1.6% to 2.0%, Mn: 0.7% to 1.0%, P≤0.020%, S≤0.005%, Al≤0.01%, N≤0.004%, Cr: 0.1% to 0.2%, and the balance is Fe and unavoidable impurity elements.

[0011] The metallographic structure of the hot-rolled pickled steel plate of the present invention is a small amount of ferrite + pearlite; according to the American Society for Testing and Materials

[0012] "ASTM E45-13 Standard Test Method for Determination of Inclusion Content in Steel" adopts method A to test inclusions in steel plates, where inclusions of type A, type B, type C, and type D are all below grade 2.0; the yield strength R of hot-rolled pickled steel plates with a thickness of 5.1 to 8.5 mm and a width of 1000 to 1300 mm is p0.2 500~600MPa, tensile strength R m 900~1050MPa, elongation after fracture 18% to 25%; the depth of single-sided decarburization layer of hot-rolled pickled steel plate is ≤1.5%×steel plate thickness; 25° bending test, d=2a is qualified.

[0013] The reasons why the chemical composition of the high plasticity hot-rolled pickled steel sheet described in the present invention is limited to the above range are as follows:

[0014] C: Carbon is the main element that affects the strength and hardness of steel plates after annealing and quenching. However, the higher the C content, the greater the decarburization tendency, and the surface hardenability and fatigue resistance will be affected to a certain extent. The C content set in the present invention is 0.56% to 0.63%.

[0015] Si: It can greatly improve the elastic limit of steel and is the most important anti-elasticity element for spring steel. In addition, because Si has a strong solid solution strengthening effect and significantly reduces the critical cooling rate of steel, it can improve the hardenability of steel. In the present application, due to the use of ultra-low Al design, Si can be used as a deoxidizer during smelting to improve the deoxidation and desulfurization capabilities. But at the same time, high-Si steel grades are prone to form iron silicate oxide scale on the surface of the steel plate during hot rolling, which greatly increases the difficulty of conventional pickling. Taking into account the product performance requirements and the consistent manufacturing process capabilities, the Si value set in the present invention is 1.6-2.0%.

[0016] Mn: Manganese is one of the most effective elements for improving hardenability and is also a solid solution strengthening element. At the same time, when properly matched with silicon, it can reduce the decarburization caused by silicon. However, too high a manganese content can easily cause slab segregation and affect the uniformity of the hot-rolled sheet structure. The present invention sets Mn to 0.7% to 1.0%.

[0017] S and P are impurity elements in steel. The lower the content, the better. Meanwhile, S is easy to form MnS in steel, which affects fatigue performance. Considering economy and feasibility, the present invention sets S≤0.005% and P≤0.020%.

[0018] Al: It is a strong oxidizing element, which can effectively reduce oxide inclusions in steel and purify steel, and is mostly used as a deoxidizer in steel. But at the same time, the Al2O3 particles formed are large and not plastic. They are difficult to deform during processing and use, which causes stress concentration and has a certain impact on the plasticity and fatigue strength of spring steel. This application adopts an ultra-low aluminum and silicon deoxidation design, which can greatly improve the purity of the steel, and the total oxygen content can be controlled below 0.004%. The present invention sets Al≤0.01%.

[0019] Cr: It is an element that can significantly improve hardenability. At the same time, it is a strong carbide-forming element that can strongly prevent the diffusion of carbon in austenite, thereby reducing the diffusion coefficient of carbon and the tendency of decarburization. At the same time, during tempering, it can make the carbides in the steel small and uniform, effectively inhibiting the structural transformation during low-temperature tempering, so the tempering stability is good. The present invention limits the Cr content to 0.1% to 0.2%.

[0020] N: It is easy to form fine nitrides with alloy elements in steel, and may precipitate secondary during the tempering process of spring steel, resulting in reduced toughness of the steel. Therefore, the present invention limits N to ≤ 0.004%.

[0021] A method for manufacturing a high-plasticity hot-rolled pickled steel plate with a thickness of 5.1 mm or more for a vehicle coil spring, the method comprising:

[0022] The molten steel is continuously cast to obtain a continuous casting slab, wherein the weight percentage of the chemical composition of the molten steel is: C: 0.56% to 0.63%, Si: 1.6% to 2.0%, Mn: 0.7% to 1.0%, P≤0.020%, S≤0.005%, Al≤0.01%, N≤0.004%, Cr: 0.1% to 0.2%, and the balance is Fe and unavoidable impurity elements; the thickness of the slab continuous casting is 210 to 230 mm, the width is 1000 to 1300 mm, and the length is 5000 to 8500 mm; the inclusion level in the continuous casting slab is controlled to be ≤ level 2.0;

[0023] The continuous casting slab is heated in a heating furnace and then hot rolled. The continuous casting slab is fed into the heating furnace by hot charging and hot delivery. The temperature of the continuous casting slab into the heating furnace is ≥400°C. The excess air coefficient in the heating furnace is 0.95-1.00. The heating temperature of the continuous casting slab is 1200-1260°C and the heating time is 180-240min. The hot rolling is a two-stage rolling process. The rough rolling is 6 continuous rollings. The rolling is performed above the austenite recrystallization temperature. The rolling end temperature is 1040-1100℃; the intermediate billet thickness after rough rolling is controlled to be 40-44mm; the finishing rolling is 7-pass continuous rolling, and the finishing temperature of the finishing rolling is 860-900℃; after finishing rolling, the thickness of the steel plate is controlled to be 5.1-8.5mm; the laminar cooling adopts front-stage cooling, and the cooling rate is 5-8℃ / s; the laminar cooling end temperature is 650-685℃; the coiling temperature is 640-675℃ to obtain the hot-rolled steel coil;

[0024] The hot rolled steel coil is slowly cooled after coiling, and the cooling rate of the hot rolled steel coil to 200°C is 7-10°C / h;

[0025] After the hot-rolled steel coil is unrolled, it is subjected to push-pull pickling, rinsing, drying and oiling, and the finished hot-rolled pickled steel plate is obtained by the coil. The push-pull pickling adopts hydrochloric acid pickling solution; the pickling temperature is 80-90°C, and the pickling time is 100-150s; the hot-rolled steel plate is sequentially pickled by the hydrochloric acid pickling solution in the first acid tank, the second acid tank and the third acid tank, and the mass concentration of the hydrochloric acid in the first acid tank, the second acid tank and the third acid tank is 80-120g / L, 120-160g / L and 100-180g / L respectively; the Fe 2+ The mass concentrations are 50-80 g / L, 20-40 g / L, and 5-10 g / L respectively; a pickling accelerator with a volume concentration of 0.8‰ to 1.2‰ is added to the hydrochloric acid pickling solution, and the pickling accelerator is a mixture of phosphate and hydroxy acid salt.

[0026] The reasons for the production process adopted by the present invention are as follows:

[0027] 1. Setting of the temperature of the continuous casting slab entering the heating furnace, the heating temperature of the continuous casting slab and the heating time

[0028] Since the steel for vehicle coil springs has a high alloy content, rapid heating from a cold state will produce certain thermal stresses that cause the slab to deform or crack. Therefore, it is usually required that the continuous casting slab be cut and then put into the furnace in a hot state. The continuous casting slab is put into the heating furnace by hot charging and hot delivery. The temperature of the continuous casting slab entering the heating furnace is ≥400°C. After the slab enters the heating furnace, it gradually austenitizes as the temperature rises, which can significantly improve the various components and organizational segregation caused by the solidification process of the slab. Generally speaking, the higher the heating temperature and the longer the insulation time, the better the uniformity of the composition and organization. However, due to the high silicon content of spring steel, the decarburization tendency will increase during the heating process, and the depth of the decarburization layer will increase accordingly, which is not conducive to the surface quality. The present invention sets the heating temperature of the continuous casting slab to 1200-1260°C and the heating time to 180-240min. Considering the influence of the atmosphere in the heating furnace on decarburization, the present invention sets the excess air coefficient in the heating furnace to 0.95-1.0.

[0029] 2. Setting of finishing temperature, laminar cooling mode, laminar cooling speed, laminar cooling ending temperature and coiling temperature

[0030] The finishing temperature setting is generally to ensure that the entire slab is fully rolled in the austenite zone to avoid mixed crystals caused by rolling in the two-phase zone at the end of rolling. The setting of the laminar cooling rate and coiling temperature after rolling mainly considers the final microstructure of the spring steel. According to research, when the cooling rate is 1℃ / s, the starting temperature Ar3 of the ferrite phase transformation is 720℃, the ending temperature Ar1 is 670℃, the starting temperature of the pearlite phase transformation is 670℃, and the ending temperature of the pearlite phase transformation is 600℃. Generally, the lower the coiling temperature, the faster the cooling rate, the smaller the size of the pearlite clusters, the finer the interlamellar spacing, the higher the strength, and the lower the plasticity. With the increase of the coiling temperature, the pearlite phase transformation is more complete, the grain size increases, and the interlamellar spacing increases. Usually, the plasticity will be improved, but the pearlite will be excessively coarsened, and the plasticity will decrease. Therefore, it is not the case that the higher the coiling temperature, the better the plasticity of the strip. At the same time, for hypoeutectoid steel with pearlite as the majority component, the morphology of pearlite and ferrite also has a significant effect on plasticity: compared with the ferrite distributed in a network between pearlite, the blocky pearlite with coarse and fine interlamellar spacing will significantly improve the plasticity. Therefore, excellent strength and plasticity require a reasonable combination of coiling temperature and cooling rate.

[0031] In addition, the coiling temperature will also have a significant impact on the thickness, structure and composition of the oxide scale of the hot-rolled steel plate. Generally speaking, the higher the coiling temperature, the thicker the oxide scale, the more alloy oxides gather at the interface between the steel base and the oxide scale, and the generated oxide scale structure is also denser. Therefore, the pickling is more difficult. Therefore, considering the strength and plasticity requirements of the final product, the temperature drop characteristics of the steel plate with a thickness of 5.1 to 8.5 mm, the impact of the increased thickness on the coiler and the subsequent pickling process requirements, this application sets the finishing temperature of the finishing rolling to 860 to 900 ° C, the laminar cooling adopts front-stage cooling control, the cooling rate is 5 to 8 ° C / S, the laminar cooling end temperature is 650 to 685 ° C; the coiling temperature is 640 to 675 ° C.

[0032] 3. Setting of slow cooling speed of hot rolled steel coil

[0033] The formulation of the cooling process for steel used in vehicle coil springs mainly considers the microstructure transformation and microstructure uniformity during the subsequent gradual cooling process after the steel plate is coiled. Studies have shown that when the cooling rate is ≤7°C / s, the microstructure of silicon-manganese spring steel is ferrite + pearlite. If the cooling rate is too fast, the residual proeutectoid ferrite will affect the hardenability. If the cooling rate is too slow, on the one hand, the production cycle will be increased and the cost will increase. At the same time, the pearlite may be over-coarsened and the plasticity will be reduced due to the longer insulation time. At the same time, too fast a cooling rate will also cause uneven internal microstructure of the steel coil to varying degrees, thereby increasing the internal stress. When shearing in strips, steel plates with a thickness of 5.1 to 8.5 mm are prone to brittle fracture or poor incision quality. Therefore, the cooling process setting of this application requires the coiling to enter the slow cooling wall or slow cooling hood for cooling as soon as possible, and the cooling rate of the hot-rolled steel coil to 200°C is 7 to 10°C / h.

[0034] 4. Setting of pickling process

[0035] Due to the high Si content in the steel used for vehicle coil springs, there is a layer of highly corrosion-resistant iron silicate in the base layer of the oxide scale, which is mixed with the pro-eutectoid product Fe3O4 of Fe0 to form a highly dense oxide layer during the slow cooling stage after hot rolling and coiling, which greatly increases the difficulty of pickling in the later stage of pickling. In addition, under the condition of long-term slow cooling, high temperature and low oxygen, the loose Fe0 generated in the layer cooling stage is almost completely converted into dense pro-eutectoid Fe3O4 and a mixed product of eutectoid Fe3O4+Fe, which greatly increases the difficulty of pickling this type of steel.

[0036] The present application has found through research that the pickling speed can be effectively improved by controlling the key components in the pickling solution at different pickling stages and adding a pickling accelerator whose main function is to strengthen the interfacial reaction. Therefore, the present application sets the following pickling process under the process conditions of push-pull hydrochloric acid pickling: push-pull pickling uses hydrochloric acid pickling solution; the pickling temperature is 80-90°C, and the pickling time is 100-150s; the hot-rolled steel plate is pickled in the first acid tank, the second acid tank, and the third acid tank in turn with the hydrochloric acid pickling solution, and the mass concentrations of the hydrochloric acid in the first acid tank, the second acid tank, and the third acid tank are 80-120g / L, 120-160g / L, and 100-180g / L, respectively; the Fe 2+ The mass concentrations are 50-80 g / L, 20-40 g / L, and 5-10 g / L respectively; a pickling accelerator with a volume concentration of 0.8‰ to 1.2‰ is added to the hydrochloric acid pickling solution, and the pickling accelerator is a mixture of phosphate and hydroxy acid salt.

[0037] The metallographic structure of the hot-rolled pickled steel plate produced by the method of the present invention is a small amount of ferrite + pearlite; according to the American Society for Testing and Materials "ASTME45-13 Standard Test Method for Determining Inclusion Content in Steel", method A is used to test the inclusions in the steel plate, wherein the inclusions of type A, type B, type C, and type D are all below grade 2.0; the yield strength R of the hot-rolled pickled steel plate with a thickness of 5.1 to 8.5 mm and a width of 1000 to 1300 mm is p0.2 500~600MPa, tensile strength R m 900~1050MPa, elongation after fracture 18% to 25%; the depth of single-sided decarburization layer of hot-rolled pickled steel plate is ≤1.5%×steel plate thickness; 25° bending test, d=2a is qualified.

[0038] Compared with the prior art, the present invention has the following positive effects: 1. The present invention adopts high Si + ultra-low aluminum design to improve the purity of steel and meet the high fatigue requirements of steel for vehicle coil springs; through the process design of hot rolling heating process, rolling process and slow cooling process, the high strength and high plasticity performance requirements of hot-rolled pickled steel plates for vehicle coil springs with a thickness of 5.1 to 8.5 mm and a width of 1000 to 1300 mm are achieved. 2. The present invention realizes the smooth unwinding of hot-rolled pickled steel plates for vehicle coil springs with a thickness of 5.1 to 8.5 mm and a width of 1000 to 1300 mm and the efficient removal of iron oxide scale on the surface of the hot-rolled steel plates through the reasonable setting of hot rolling process and pickling process. BRIEF DESCRIPTION OF THE DRAWINGS

[0039] Figure 1 This is a metallographic photograph of the hot-rolled pickled steel plate according to Example 1 of the present invention. DETAILED DESCRIPTION

[0040] The present invention is further described below in conjunction with Examples 1-6, as shown in Tables 1-7.

[0041] Table 1 shows the chemical composition (by weight percentage) of the steel according to the embodiment of the present invention, with the remainder being Fe and unavoidable impurities.

[0042] Table 1 Chemical composition of the steel of the present invention, unit: weight percentage.

[0043]

[0044] The molten steel meeting the chemical composition requirements is obtained by converter smelting, the molten steel is subjected to Ar blowing treatment in the LF ladle refining furnace refining process, the RH furnace is subjected to vacuum cycle degassing treatment and composition fine-tuning, the molten steel is subjected to Ar blowing protection throughout the process and continuously cast to obtain a continuous casting slab with a thickness of 210-230mm, a width of 1000-1300mm, and a length of 5000-8500mm; the inclusion level in the continuous casting slab is controlled to be ≤level 2.0.

[0045] The continuous casting slab is directly sent to the heating furnace for heating. The continuous casting slab is sent to the heating furnace by hot charging and hot delivery. The continuous casting slab is heated in the heating furnace and then hot rolled. The hot rolling is a two-stage rolling process. The rolling is controlled by the rough rolling and finishing rolling mills. After laminar cooling, the coiling is carried out. The laminar cooling adopts the front stage cooling to produce qualified hot rolled steel coils. The thickness of the hot rolled steel plate is 5.1-8.5mm. The heating furnace process control parameters are shown in Table 2; the hot rolling process control parameters are shown in Table 3.

[0046] Table 2 Process control parameters of the heating furnace in the embodiment of the present invention

[0047]

[0048] Table 3 Hot rolling process control parameters of the embodiment of the present invention

[0049]

[0050]

[0051] After the hot rolled steel coil is coiled, it enters the slow cooling hood for slow cooling. The hot rolled steel coil slow cooling process control parameters are shown in Table 4.

[0052] Table 4 Control parameters of the slow cooling process of hot-rolled steel coil according to the embodiment of the present invention

[0053] Hot rolling parameters Cooling speed / ℃ / h The present invention 7-10 Example 1 8.9 Example 2 8.6 Example 3 7.9 Example 4 7.3 Example 5 7.4 Example 6 7.7

[0054] The hot rolled steel coil is unrolled and subjected to push-pull pickling, rinsing, drying and oiling to obtain a finished hot rolled pickled steel plate. The push-pull pickling adopts hydrochloric acid pickling solution. The pickling process control parameters are shown in Tables 5 and 6.

[0055] Table 5 Pickling process control parameters of the embodiment of the present invention (I)

[0056]

[0057] Table 6 Pickling process control parameters of the present invention (II)

[0058]

[0059] The hot-rolled pickled steel sheet obtained by the above method is shown in Figure 1 The metallographic structure of the hot-rolled pickled steel plate is a small amount of ferrite + pearlite; according to the American Society for Testing and Materials "ASTME45-13 Standard Test Method for Determination of Inclusion Content in Steel", method A is used to test the inclusions in the steel plate. Among them, the inclusions of type A, type B, type C, and type D are all below level 2.0; the yield strength R of the hot-rolled pickled steel plate with a thickness of 5.1 to 8.5 mm and a width of 1000 to 1300 mm is p0.2 500~600MPa, tensile strength R m 900~1050MPa, elongation after fracture 18% to 25%; the depth of single-sided decarburization layer of hot-rolled pickled steel plate is ≤1.5%×steel plate thickness; 25° bending test, d=2a is qualified, and there is no cracking.

[0060] The hot-rolled pickled steel plate obtained by the present invention was subjected to a tensile test according to "GB / T228.1-2010 Tensile test of metallic materials Part 1: Room temperature test method", cold bending was performed according to "GB / T232-2010 Bend test method of metallic materials", inclusion rating was performed using method A according to "ASTME45-13 Standard test method for determination of inclusion content in steel", and decarburization layer thickness was measured according to "GB / T 224-2008 Method for determination of decarburization layer depth of steel", and its performance parameters are shown in Table 7.

[0061] Table 7 Performance parameters of hot-rolled pickled steel plates according to the present invention

[0062]

[0063] In Table 7, the elongation after fracture of the hot-rolled pickled steel plate was tested using S0 is the cross-sectional area of ​​the sample steel plate.

[0064] In addition to the above embodiments, the present invention may also have other implementation modes. Any technical solution formed by equivalent replacement or equivalent transformation falls within the protection scope required by the present invention.

Claims

1. A high plasticity hot-rolled pickled steel plate with a thickness of 5.1 mm or more for vehicle coil springs, wherein the chemical composition by weight is: C: 0.56% to 0.63%, Si: 1.6% to 2.0%, Mn: 0.7% to 1.0%, P ≤ 0.020%, S ≤ 0.005%, Al ≤ 0.01%, N ≤ 0.004%, Cr: 0.1% to 0.2%, and the balance is Fe and unavoidable impurity elements; according to the American Society for Testing and Materials "ASTM E45-13 Standard Test Method for Determination of Inclusion Content in Steel" adopts Method A to test inclusions in steel plates, among which: Inclusions of type A, B, C, and D are all below level 2.0; the yield strength R of hot-rolled pickled steel plates with a thickness of 5.1 to 8.5 mm and a width of 1000 to 1300 mm is p0.2 500~600MPa, tensile strength R m 900~1050MPa, elongation after fracture 18% to 25%; the depth of single-sided decarburization layer of hot-rolled pickled steel plate is ≤1.5%×steel plate thickness; 25° bending test, d=2a is qualified.

2. The high plasticity hot-rolled pickled steel sheet with a thickness of 5.1 mm or more for vehicle coil springs according to claim 1, characterized in that: The metallographic structure of hot-rolled pickled steel plate is a small amount of ferrite + pearlite.

3. The method for manufacturing a high-plasticity hot-rolled pickled steel sheet with a thickness of 5.1 mm or more for a vehicle coil spring according to claim 1, characterized in that: The following steps are involved: The molten steel is continuously cast to obtain a continuous casting slab, wherein the weight percentage of the chemical composition of the molten steel is: C: 0.56% to 0.63%, Si: 1.6% to 2.0%, Mn: 0.7% to 1.0%, P≤0.020%, S≤0.005%, Al≤0.01%, N≤0.004%, Cr: 0.1% to 0.2%, and the balance is Fe and unavoidable impurity elements; the thickness of the slab continuous casting is 210 to 230 mm, the width is 1000 to 1300 mm, and the length is 5000 to 8500 mm; the inclusion level in the continuous casting slab is controlled to be ≤ level 2.0; The continuous casting slab is heated in a heating furnace and then hot rolled. The continuous casting slab is fed into the heating furnace by hot charging and hot delivery. The temperature of the continuous casting slab into the heating furnace is ≥400°C. The excess air coefficient in the heating furnace is 0.95-1.

00. The heating temperature of the continuous casting slab is 1200-1260°C and the heating time is 180-240min. The hot rolling is a two-stage rolling process. The rough rolling is 6 continuous rollings. The rolling is performed above the austenite recrystallization temperature. The rolling end temperature is 1040-1100℃; the intermediate billet thickness after rough rolling is controlled to be 40-44mm; the finishing rolling is 7-pass continuous rolling, and the finishing temperature of the finishing rolling is 860-900℃; after finishing rolling, the thickness of the steel plate is controlled to be 5.1-8.5mm; the laminar cooling adopts front-stage cooling, and the cooling rate is 5-8℃ / s; the laminar cooling end temperature is 650-685℃; the coiling temperature is 640-675℃ to obtain the hot-rolled steel coil; The hot rolled steel coil is slowly cooled after coiling, and the cooling rate of the hot rolled steel coil to 200°C is 7-10°C / h; After the hot-rolled steel coil is unrolled, it is subjected to push-pull pickling, rinsing, drying and oiling, and the finished hot-rolled pickled steel plate is obtained by the coil. The push-pull pickling adopts hydrochloric acid pickling solution; the pickling temperature is 80-90°C, and the pickling time is 100-150s; the hot-rolled steel plate is sequentially pickled by the hydrochloric acid pickling solution in the first acid tank, the second acid tank and the third acid tank, and the mass concentration of the hydrochloric acid in the first acid tank, the second acid tank and the third acid tank is 80-120g / L, 120-160g / L and 100-180g / L respectively; the Fe 2+ The mass concentrations are 50-80 g / L, 20-40 g / L, and 5-10 g / L respectively; a pickling accelerator with a volume concentration of 0.8‰ to 1.2‰ is added to the hydrochloric acid pickling solution, and the pickling accelerator is a mixture of phosphate and hydroxy acid salt.

Images