Low-cost Ti microalloyed high-surface-quality automobile steel and production method thereof

Abstract

The invention belongs to the field of metal materials, and particularly relates to low-cost Ti microalloyed high-surface-quality automobile steel and a production method thereof. The steel comprises the following chemical components in percentage by weight: 0.66%-0.83% of Si, 0.516%-0.718% of Al, 0.516%-0.718% of Ti, Ce and the balance of Fe and inevitable impurities. According to the invention, the conventional component with higher Si content is adopted, the Si content does not need to be specially reduced, the smelting is easy, the cost is low, the surface quality is good, oxide scale stripes and chromatic aberration defects are avoided, and the surface roughness Ra is 1.30-1.80 microns.

Description

Technical Field

[0001] This invention belongs to the field of metallic materials, and specifically relates to a low-cost Ti microalloyed high surface quality automotive steel and its production method. Background Technology

[0002] With the rapid development of the automotive industry, the demands for lightweighting, safety, and cost reduction are increasing. Hot-rolled pickled steel sheet is made from high-quality hot-rolled coils through processes such as uncoiling, pickling, rinsing, drying, edge trimming, and oiling. Hot-rolled pickled steel sheet offers a high cost-performance ratio, falling between cold-rolled and hot-rolled steel sheets. Its surface quality and application requirements are between those of hot-rolled and cold-rolled steel sheets, making it an ideal substitute for some types of both. Hot-rolled pickled steel sheet is widely used in automotive chassis, wheels, and body structural parts. Currently, given the challenging steel market conditions, it is one of the most profitable products for steel companies, effectively reducing procurement costs for users while meeting their requirements.

[0003] Chinese patent application CN111926252A discloses a hot-rolled pickled steel sheet for deep drawing and its production method. The hot-rolled pickled steel sheet produced by adding a certain amount of Ti, Cu and B to the ordinary C-Mn composition system has a yield strength of 210-260 MPa, a tensile strength of ≥330 MPa and an elongation A50 of ≥45%. Due to the high cost of adding Ti, Cu and B and the low elongation after fracture, the Si content is low, which provides a certain guarantee for the surface quality of the steel sheet. However, it ignores the risks of reduced strength of the ferrite matrix and the formation of inclusions in the steel.

[0004] Chinese patent application CN114351033A discloses a method for manufacturing a low-alloy QStE420TM hot-rolled pickled automotive steel sheet. The method uses a common C-Mn composition system to design and add a certain amount of Nb and Ti, with Si ≤ 0.50% to produce a hot-rolled pickled steel sheet with an elongation after fracture ≥ 16%. However, due to the high Si content, the surface quality is prone to oxidation streaks and color difference defects, and the elongation after fracture is low, which does not meet the requirements of high surface quality and high formability passenger car chassis structural parts. Summary of the Invention

[0005] To address the development needs in the automotive steel industry, this invention presents a low-cost Ti microalloyed high-surface-quality automotive steel and its production method. The steel exhibits a yield strength ≥270MPa, tensile strength ≥425MPa, longitudinal elongation A ≥47%, hole expansion rate ≥135%, and meets the acceptable longitudinal cold bending requirement of 180° (D=a). It utilizes a conventional composition with a high Si content, eliminating the need for special Si reduction, making it easy to smelt, cost-effective, and possessing excellent surface quality with no oxide scale streaks or color differences. The surface roughness Ra is 1.30–1.80 μm.

[0006] To achieve the above objectives, the present invention employs the following technical solution: A low-cost Ti microalloyed high surface quality automotive steel has the following chemical composition by weight percentage: C: 0.045%–0.080%, Si: 0.66%–0.83%, Mn: 1.32%–1.46%, Al: 0.516%–0.718%, Ti: 0.0087%–0.0187%, Ce: 0.0030%–0.0080%, with P ≤ 0.010%, S ≤ 0.004%, N ≤ 0.003%, and the balance being Fe and unavoidable impurities.

[0007] The microstructure of the finished steel plate is 96%–99% ferrite, 1%–4% pearlite, and 0%–3% bainite.

[0008] The steel plate has a yield strength ≥270MPa, tensile strength ≥425MPa, longitudinal elongation A ≥47%, hole expansion rate ≥135%, and longitudinal cold bending of 180° D=a is qualified (the diameter of the bending mandrel is equal to the thickness of the steel plate).

[0009] The surface roughness Ra of the steel plate is 1.30–1.80 μm. The roughness of the steel plate in this invention, while meeting the surface quality requirements for steel plates, can increase the frictional resistance during welding, which is beneficial for heat transfer and fusion during welding, improving weld quality. During spot welding, it can keep the contact resistance within a reasonable range, avoiding incomplete welds or burn-through due to poor contact, and improving weld strength and stability. It can also increase the friction coefficient of the friction surface, improving the anti-slip ability of the connection, thereby improving the overall stability of the structure.

[0010] C: Carbon is a common strengthening element in steel. Interstitial solid-solution carbon atoms cause certain lattice distortion in the matrix, playing a role in solid-solution strengthening. It also combines with Ti to form submicron or nano-sized fine spherical TiC particles for dispersed precipitation strengthening, refining the grains and strengthening the matrix. In this invention, carbon ensures the strength, formability, and hole-expanding properties of the steel plate. Too low a carbon content will not achieve the mechanical properties of the steel plate in this invention, while too high a content will cause the steel plate to become brittle, posing a risk of delayed fracture and hot-rolling edge cracking, and also negatively impacting the weldability, plasticity, and toughness of the steel plate. In this invention, the overall carbon content is required to be within a low range, which helps reduce the risk of delayed fracture and hot-rolling edge cracking, and is also beneficial to the weldability of the steel plate. Therefore, the optimal range of carbon in this invention is 0.045% to 0.080%.

[0011] Si: Silicon is one of the key elements in this invention. Silicon dissolves in the crystal lattice, producing a solid solution strengthening effect, increasing the strength and yield point of the steel sheet, while maintaining good ductility, which is particularly important for high-strength passenger vehicle chassis components. It can also refine the grains, improving the uniform elongation, local elongation, and porosity of the steel sheet, making it less prone to cracking and wrinkling during stamping, which is beneficial for manufacturing complex passenger vehicle chassis components. Therefore, the silicon content in this invention is 0.66% to 0.83%.

[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, delaying the transformation of austenite to pearlite, and lowering the transformation temperature of bainite. However, excessive Mn content leads to Mn segregation, which can reduce the plasticity of steel, worsen the uniformity of steel plate structure during hot rolling, and easily cause severe banded structural defects in the structure, which is not conducive to the expansion performance. Therefore, after comprehensive consideration, the present invention selects a manganese content of 1.32% to 1.46%.

[0013] P: Phosphorus is an impurity element in steel. It tends to agglomerate at grain boundaries. When the phosphorus content in steel is high, Fe2P particles are easily formed, which reduces the plasticity, toughness and porosity of the steel. Therefore, the lower its content, the better. In order to obtain a higher elongation, its upper limit is set at 0.010%.

[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. It seriously affects the plasticity, formability and hole expansion performance of steel plates. Therefore, the lower the content, the better. The upper limit is set at 0.004%.

[0015] Al: When the Si content in steel is high, a red iron oxide scale, mainly composed of Fe2O3, will form on the surface of the steel plate during rolling. This is because Si promotes the formation of Fe-Si-O composite oxides, such as Fe2SiO4, at the interface between the iron oxide scale and the matrix. Fe2SiO4 will penetrate into the space between FeO and the matrix in a liquid state, and after solidification, it will exhibit an anchor-like morphology, firmly anchoring the FeO layer and making the iron oxide scale difficult to remove. The unremoved FeO layer will be crushed during subsequent hot rolling, increasing the contact area with air and accelerating the transformation process of FeO→Fe3O4→Fe2O3, ultimately forming a red iron oxide scale mainly composed of Fe2O3. Furthermore, during subsequent pickling, the steel plate surface will have iron oxide scale streaks and color difference defects. The role of adding Al in this invention is that Al's oxidation activity (i.e., its affinity for oxygen) is much higher than that of Si. Al is a strong deoxidizing element, and its ability to combine with oxygen is stronger at high temperatures. It will preferentially react with oxygen in the steel or in the air to first generate Al2O3. Si has relatively low oxidizing activity. Only after the oxidation reaction of Al is basically completed (or the oxygen supply is sufficient) will it gradually combine with oxygen to form iron-silicon composite oxides (such as Fe2SiO4). Therefore, in the process of iron oxide scale formation, the oxidation reaction of Al occurs before Si, and Al2O3 will be generated before the oxide of Si, avoiding the formation of Fe2SiO4. This is beneficial for the absence of iron oxide scale streaks and color difference defects on the steel plate surface during subsequent pickling. Therefore, in this invention, the Al content is limited to 0.516% to 0.718%.

[0016] Ti: Titanium can effectively delay the recrystallization of deformed austenite, prevent austenite grain growth, increase the austenite recrystallization temperature, refine grains, and improve the strength and toughness of steel. Since free N atoms in steel deteriorate the toughness of steel plates, Ti combines with impurity elements N in steel to form blocky or angular TiN. Therefore, the formation of TiN has a solidifying effect on N. However, if the N content is too high, the size of the blocky or angular TiN will be too large, which will deteriorate the performance of the steel plate and make the toughness of the weld heat-affected zone and the fatigue performance of the steel plate worse. Therefore, this invention limits N to ≤0.003% to avoid the formation of too much blocky or angular TiN. Furthermore, the purpose of Ti microalloying in this invention is to enable Ti and C to form submicron or nano-sized fine spherical TiC particles for dispersed precipitation strengthening, refining the grains, strengthening the matrix, and playing a role in grain refinement strengthening and precipitation strengthening. It can also strengthen ferrite, which is beneficial to obtaining excellent mechanical properties, porosity expansion properties and fatigue properties, thereby improving the service life of steel in passenger car chassis structural components. Therefore, the optimal range of Ti content in this invention is between 0.0087% and 0.0187%.

[0017] Ce: Cerium has a strong affinity for harmful elements such as oxygen and sulfur in steel, forming stable oxides (e.g., Ce₂O₃) and sulfides (e.g., CeS). These compounds have high melting points and are easily removed from molten steel by flotation, reducing non-metallic inclusions and improving steel purity. For inclusions that cannot be completely removed, cerium can alter their morphology and distribution, transforming brittle, elongated sulfides into spherical or dot-shaped cerium sulfides, reducing the adverse effects of inclusions on the mechanical properties of steel, especially improving toughness and fatigue strength. The addition of cerium can inhibit grain growth during heating or cooling, refining austenite or ferrite grains, thereby improving the strength and toughness of the steel. Through purification and modification, it enhances the plasticity and toughness of steel and reduces the risk of cracking during hot and cold working. It can also improve the weldability and oxidation resistance of steel, thus increasing its service life at high temperatures. Therefore, this invention limits the Ce content to 0.0030%–0.0080%.

[0018] N: For the N content in steel, the lower the N content, the better. However, too low a content will lead to production difficulties and increased costs. It is also necessary to ensure a certain amount of AlN formation to refine the grains, improve the strength and elongation of the steel plate, and avoid forming too much blocky or angular TiN. Therefore, the N content in this invention is ≤0.003%.

[0019] A low-cost Ti microalloyed high surface quality automotive steel production method includes smelting and continuous casting, hot rolling, and pickling processes, specifically including: (1) Smelting and continuous casting process: The raw material is pretreated with KR hot metal to control the S content to be less than 0.004%. After slag removal, it enters the converter. In the converter smelting, the double slag method is used to remove P, and the P content is controlled to be ≤0.010%. At the end of the converter smelting, the C content is controlled to be 0.045~0.080%. Argon gas is blown for more than 4.5 minutes when tapping the steel (the argon blowing and killing 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, and then the slab is continuously cast. The superheat of continuous casting is ≤20℃, and the continuous casting drawing speed is 1.1~1.6m / m. (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 150-250A, and the secondary cooling water volume to 0.75-1.25L / 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 3.0-3.5mm (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).

[0020] (2) Hot rolling process: The continuously cast slab with a thickness of (170~230) mm and a width of (1060~2100) mm is loaded into a walking beam furnace for heating at a temperature of 1182~1222℃ and a holding time of 120~180 min. The roughing process adopts a 3+3 rolling mode (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℃. The thickness of the intermediate slab is 38.0~58.0 mm and the width is 1060~2100 mm. The intermediate slab is insulated with a heat preservation 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 1030~1080℃. The final rolling temperature is 875–935℃. After 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 635–676℃, air cooling is performed for 13–18 seconds, followed by rapid cooling at a rate greater than 22℃ / s. After rapid cooling to 518–568℃, the steel is coiled. The purpose of laminar flow cooling to 635–676℃ followed by air cooling for 13–18 seconds is to rapidly generate ferrite, inhibiting grain growth while ensuring the ferrite content, thus refining the ferrite grains. Rapid cooling to 518–568℃ aims to rapidly generate a small amount of bainite, inhibiting grain growth while ensuring the bainite content, thus refining the bainite grains. The final microstructure of the rolled steel plate has a ferrite volume percentage of 96%–99%, a pearlite volume percentage of 1%–4%, and a bainite volume percentage of 0%–3%.

[0021] (3) After coiling, heat to 295-396℃ in a bell-type furnace, hold for 5.0-8.0 min, and then cool with the furnace. The purpose is to release residual stress. During hot rolling, uneven temperature and plastic deformation will cause residual stress inside the steel plate. At 295-396℃, atoms gain a small amount of energy and can release stress through dislocation movement, avoiding warping due to stress release when the steel plate is uncoiled and cut, thus optimizing the plate shape. It can also reduce the wear of shearing tools during processing, indirectly improving processing efficiency and reducing production material costs. In addition, the reaction rate of iron and oxygen is extremely slow in this temperature range. Combined with the nitrogen protective atmosphere commonly used in bell-type furnaces, it can minimize the formation of iron oxide scale, preserve the original surface state of the steel plate, and reduce the difficulty and cost of subsequent pickling surface treatment.

[0022] (4) Pickling process: Hydrochloric acid is used to continuously clean the iron oxide scale of the steel plate through the coil. After hot rolling, the steel plate is uncoiled and pickled on the pickling line. Before pickling, the steel plate is tension leveled with an elongation of 0.5% 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 52 to 72 g / L and the temperature of the pickling solution in tank 1 is 72.0 to 82.0℃. The concentration of the pickling solution in tank 2 is 77 to 102 g / L and the temperature of the pickling solution in tank 2 is 70.0 to 75.0℃. The concentration of the pickling solution in tank 3 is 110 to 130 g / L and the temperature of the pickling solution in tank 3 is 55.0 to 75.0℃. The concentration of the pickling solution in tank 4 is 135 to 145 g / L and the temperature of the pickling solution in tank 4 is 50.0 to 60.0℃. During pickling, a corrosion inhibitor is added to the acid solution, with the inhibitor accounting for 0.11–0.16% of the pickling solution by weight. The rinsing water temperature is 55–65℃, and the pickling and rinsing speeds are controlled at 80–155 m / min. The pickling process is carried out under tension, with a tension of 35–65 kN. Finally, the product is coated with oil and rolled up to obtain the finished product, which has a thickness of 1.50–6.00 mm and a surface roughness Ra of 1.30–1.80 μm.

[0023] Compared with the prior art, the beneficial effects of the present invention are: 1. The purpose of adding Al in this invention is that Al's oxidizing activity (i.e., its affinity for oxygen) is much higher than that of Si. Al is a strong deoxidizing element, and its ability to combine with oxygen is even stronger at high temperatures. It will preferentially react with oxygen in the steel or in the air to first generate Al2O3. Si, on the other hand, has relatively low oxidizing activity. Only after the oxidation reaction of Al is basically complete (or the oxygen supply is sufficient) will it gradually combine with oxygen to form iron-silicon composite oxides (such as Fe2SiO4). Therefore, in the process of iron oxide scale formation, the oxidation reaction of Al occurs before that of Si, and Al2O3 will be generated before the oxide of Si, avoiding the formation of Fe2SiO4. This is beneficial for the subsequent pickling process, which is to prevent iron oxide scale streaks and color difference defects on the steel plate surface.

[0024] 2. Titanium can effectively delay the recrystallization of deformed austenite, prevent austenite grain growth, increase the austenite recrystallization temperature, refine the grains, and improve the strength and toughness of steel. Since free nitrogen atoms in steel deteriorate the toughness of the steel plate, Ti combines with impurity elements such as nitrogen to form blocky or angular TiN. Therefore, the formation of TiN has a nitrogen-fixing effect. However, excessive nitrogen content leads to excessively large blocky or angular TiN sizes, deteriorating the steel plate's properties and worsening the toughness of the weld heat-affected zone and the fatigue performance of the steel plate. Therefore, this invention limits N to ≤0.003% to avoid the formation of excessive blocky or angular TiN. Furthermore, the purpose of Ti microalloying in this invention is to enable Ti and C to form submicron or nanoscale fine spherical TiC particles for dispersed precipitation strengthening, refining the grains, strengthening the matrix, and achieving both grain refinement strengthening and precipitation strengthening. It can also strengthen ferrite, which is beneficial for obtaining excellent mechanical properties, porosity, and fatigue performance, thereby improving the service life of steel in passenger car chassis structural components.

[0025] 3. Cerium has a strong affinity for harmful elements such as oxygen and sulfur in steel, forming stable oxides (e.g., Ce₂O₃) and sulfides (e.g., CeS). These compounds have high melting points and are easily removed from molten steel by flotation, reducing non-metallic inclusions and improving steel purity. For inclusions that cannot be completely removed, cerium can alter their morphology and distribution, transforming brittle, elongated sulfides into spherical or dot-shaped cerium sulfides, reducing the adverse effects of inclusions on the mechanical properties of steel, especially improving toughness and fatigue strength. The addition of cerium can inhibit grain growth during heating or cooling, refining austenite or ferrite grains, thereby improving the strength and toughness of the steel. Through purification and modification, it enhances the plasticity and toughness of steel and reduces the risk of cracking during hot and cold working. It can also improve the weldability and oxidation resistance of steel, thus extending its service life at high temperatures.

[0026] 4. The microstructure of the steel of this invention consists of ferrite, pearlite and bainite, which significantly improves the hole expansion performance of the steel plate during the forming process.

[0027] 5. This invention produces a steel plate with excellent mechanical properties: yield strength ≥270MPa, tensile strength ≥425MPa, longitudinal elongation A ≥47%, hole expansion rate ≥135%, longitudinal cold bending at 180° D=a is qualified, and surface roughness Ra is 1.30~1.80μm. It uses a conventional high Si content composition design, eliminating the need for special Si content reduction, making it easy to smelt, low in cost, and producing a good surface quality free of iron oxide scale streaks and color difference defects. Detailed Implementation

[0028] 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.

[0029] The specific embodiments of the present invention are as follows: The smelting raw materials undergo KR hot metal pretreatment to control the sulfur content below 0.004%. 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 ≤0.010%. At the end of the converter smelting, the carbon content is controlled between 0.045% and 0.080%. Argon gas is blown for at least 4.5 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 15 minutes. For slabs with a diameter of n or above, calcium treatment is performed in the refining LF process, followed by continuous casting of slabs. The superheat of continuous casting is ≤20℃, the casting speed is 1.1~1.6m / min, the electromagnetic stirring current intensity in the secondary cooling zone is controlled at 150A~250A, the secondary cooling water volume is 0.75L / kg~1.25L / kg, and light pressure is applied in the horizontal section of the secondary cooling zone, i.e., at the end of solidification, with a reduction of 3.0~3.5mm for the continuous casting slab. The slabs are stacked for more than 36 hours after being taken off the line. A continuously cast slab with a thickness of (170–230) mm and a width of (1060–2100 mm) is loaded into a walking beam furnace for heating at a temperature of 1182–1222℃ for a holding time of 120–180 min. The roughing rolling process uses a 3+3 rolling mode (R1 is rolled in 3 passes, R2 in 3 passes), for a total of 6 passes. The roughing mill exit temperature is ≥1020℃. The intermediate slab has a thickness of 38.0–58.0 mm and a width of 1060–2100 mm. An insulation cover is used to protect the intermediate slab before it enters the hot finishing mill. Temperature control is implemented to reduce the temperature drop of intermediate billets on the delayed roller table and the temperature difference between the head and tail and in the width direction. Finishing rolling is a 7-stand continuous rolling process. High-pressure water descaling is performed before finishing rolling. The entry temperature of the finishing mill is 1030–1080℃, and the final rolling temperature is 875–935℃. After 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 635–676℃, air cooling is performed. After 13–18 seconds of air cooling, rapid cooling is performed at a rate greater than 22℃ / s. After rapid cooling to 518–568℃, the billet is coiled. After hot rolling and coiling, the billet is heated to 295–396℃ in a bell-type furnace, held for 5.0–8.0 minutes, and then cooled in the furnace. After cooling, the steel plates are uncoiled and pickled on the pickling line. Before pickling, the steel plates are tension-straightened with an elongation of 0.5-1.2%. The pickling solution is hydrochloric acid. The pickling tanks are divided into four tanks: Tank 1 has a concentration of 52-72 g / L and a pickling solution temperature of 72.0-82.0℃; Tank 2 has a concentration of 77-102 g / L and a pickling solution temperature of 70.0-75.0℃; Tank 3 has a concentration of 110-130 g / L and a pickling solution temperature of 55.0-75.0℃; and Tank 4 has a concentration of 135-145 g / L and a pickling solution temperature of 50.0-60.0℃.During pickling, a corrosion inhibitor is added to the acid solution, with the inhibitor accounting for 0.11–0.16% of the pickling solution by weight. The rinsing water temperature is 55–65℃, and the pickling and rinsing speeds are controlled at 80–155 m / min. The pickling process is carried out under tension, with a tension of 35–65 kN. Finally, the product is coated with oil and rolled up to obtain the finished product, which has a thickness of 1.50–6.00 mm and a surface roughness Ra of 1.30–1.80 μm.

[0030] The specific components, smelting process, hot rolling process, bell-type furnace heating process, pickling process, steel plate properties, and volume percentage of the six embodiments of the present invention are shown in Tables 1-6.

[0031] Table 1 Chemical composition (wt, %) of the embodiments of the present invention: Table 2 Smelting process of the present invention embodiment: Table 3 Hot rolling process and bell-type furnace heating process of the present invention: Table 4. Pickling process parameters of the present invention embodiments: Table 5 Mechanical performance parameters of embodiments of the present invention: Table 6. Percentage of tissue volume in the embodiments of the present invention: .

Claims

1. A low-cost Ti microalloyed high surface quality automotive steel, characterized in that, The chemical composition of the steel, by weight percentage, is as follows: C: 0.045%–0.080%, Si: 0.66%–0.83%, Mn: 1.32%–1.46%, Al: 0.516%–0.718%, Ti: 0.0087%–0.0187%, Ce: 0.0030%–0.0080%, with P ≤ 0.010%, S ≤ 0.004%, N ≤ 0.003%, and the balance being Fe and unavoidable impurities. The aforementioned low-cost Ti microalloyed high surface quality automotive steel production method includes smelting and continuous casting, hot rolling, bell-type furnace heating and cooling, and pickling processes, specifically including: In the hot rolling process, the finishing mill inlet temperature is 1030-1080℃, the final rolling temperature is 875-935℃, and 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 635-676℃, air cooling is performed. After air cooling for 13-18s, rapid cooling is performed. The rapid cooling rate is greater than 22℃ / s. After rapid cooling to 518-568℃, the coil is then formed. After winding, the coil is heated to 295–396°C in a bell-type furnace, held for 5.0–8.0 minutes, and then cooled with the furnace.

2. The low-cost Ti microalloyed high surface quality automotive steel according to claim 1, characterized in that, The microstructure of the finished steel plate is 96%–99% ferrite, 1%–4% pearlite, and 0%–3% bainite.

3. The low-cost Ti microalloyed high surface quality automotive steel according to claim 1, characterized in that, The steel plate has a yield strength ≥270MPa, tensile strength ≥425MPa, longitudinal elongation A ≥47%, hole expansion rate ≥135%, and longitudinal cold bending 180° D=a is qualified.

4. The low-cost Ti microalloyed high surface quality automotive steel according to claim 1, characterized in that, The surface roughness Ra of the steel plate is 1.30 to 1.80 μm.

5. A method for producing low-cost Ti microalloyed high surface quality automotive steel as described in any one of claims 1-4, characterized in that, This includes smelting and continuous casting, hot rolling, bell-type furnace heating and cooling, and pickling processes. Specific details include: In the hot rolling process, the finishing mill inlet temperature is 1030-1080℃, the final rolling temperature is 875-935℃, and 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 635-676℃, air cooling is performed. After air cooling for 13-18s, rapid cooling is performed. The rapid cooling rate is greater than 22℃ / s. After rapid cooling to 518-568℃, the coil is then formed. After winding, the coil is heated to 295–396°C in a bell-type furnace, held for 5.0–8.0 minutes, and then cooled with the furnace.

6. The method for producing low-cost Ti microalloyed high surface quality automotive steel according to claim 5, characterized in that, In the smelting and continuous casting process: argon gas is blown for more than 4.5 minutes when tapping steel from the converter, RH vacuum degassing is maintained for more than 15 minutes, the superheat of continuous casting is ≤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 150~250A, the secondary cooling water volume is 0.75~1.25L / kg, a light pressure is applied at the end of solidification, the continuous casting billet reduction is 3.0~3.5mm, and the billets are stacked for more than 36 hours after leaving the line.

7. The method for producing low-cost Ti microalloyed high surface quality automotive steel according to claim 5, characterized in that, The hot rolling process also includes: a heating temperature of 1182~1222℃, a holding time of 120~180min, a roughing mill exit temperature ≥1020℃, an intermediate billet thickness of 38.0~58.0mm, a width of 1060~2100mm, and an insulation cover for the intermediate billet before it enters the hot rolling finishing mill.

8. The method for producing low-cost Ti microalloyed high surface quality automotive steel according to claim 5, characterized in that, The pickling process includes: pre-pickling straightening of the steel plate with an elongation of 0.5%–1.2%; four pickling tanks: tank 1 has a concentration of 52–72 g / L and a pickling solution temperature of 72.0–82.0℃; tank 2 has a concentration of 77–102 g / L and a pickling solution temperature of 70.0–75.0℃; tank 3 has a concentration of 110–130 g / L and a pickling solution temperature of 55.0–75.0℃; and tank 4 has a concentration of 135–145 g / L and a pickling solution temperature of 50.0–60.0℃. During pickling, a corrosion inhibitor is added to the acid solution, with the inhibitor accounting for 0.11%–0.16% of the pickling solution by weight. The rinsing water temperature is 55–65℃, and the pickling and rinsing speeds are controlled at 80–155 m / min.

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