High formability pickled-free hot-dip galvanized steel for automobiles and method for producing the same

Abstract

The present application belongs to the field of metal materials, and particularly relates to a high-formability pickling-free hot galvanizing automobile steel and a production method thereof. The steel plate of the present application is mainly suitable for manufacturing automobile structural parts. The steel plate of the present application is mainly suitable for manufacturing automobile structural parts. The chemical composition of the steel comprises C, Si, Mn, Al, Ti, V, Ge, Pr, and the balance of Fe and inevitable impurities. The yield strength of the steel plate is greater than or equal to 308 MPa, the tensile strength is greater than or equal to 448 MPa, the transverse elongation A is greater than or equal to 40%, the hole expansion ratio is greater than or equal to 121%, the transverse cold bending 180 D=a is qualified, the surface quality is good, there is no color difference defect, the surface roughness Ra is 0.7-1.4 microns. Moreover, the steel plate is obtained by using a hot rolling + reduction descaling + continuous annealing galvanizing + finishing process, instead of a conventional hot rolling + pickling + cold rolling + continuous annealing galvanizing + finishing process, so that the pickling + cold rolling process is reduced, the production cost is low, and the environment is not polluted.

Description

Technical Field

[0001] This invention belongs to the field of metallic materials, and specifically relates to a high-formability, acid-free, hot-dip galvanized automotive steel and its production method. The steel sheet of this invention is mainly suitable for manufacturing automotive structural components. Background Technology

[0002] The automotive, steel, petrochemical, and construction industries are collectively known as the four pillars of the national economy. Among them, the scale of the automotive and steel industries is a core indicator of a country's industrial development level, and major industrial powers worldwide have prioritized the automotive industry as a pillar sector of their national economies. Simultaneously, the automotive industry is a core application area for steel products, and the research, development, and application of steel products are deeply intertwined with the development trends of the automotive industry. Currently, my country's automotive industry is entering a stage of rapid development, and automotive steel is upgrading towards lower costs, higher strength, green environmental protection, and higher safety. To reduce the weight of automotive structural components and achieve energy conservation, emission reduction, and environmental protection goals, the application scenarios of high-performance, high-surface-quality automotive steel sheets are continuously expanding, and their adoption is accelerating. Currently, the production of most automotive galvanized steel sheets requires pickling and cold rolling, which not only pollutes the environment but also incurs high costs for waste acid treatment, and the cold rolling process also increases production costs.

[0003] Chinese patent application CN 115652207A discloses a short-process economical cold-rolled DH steel sheet of 780MPa grade and its production method. It uses a common C-Mn composition system and adds a certain amount of Ti, Mg and Ca to produce a hot-rolled + pickling + cold-rolled + continuous annealing + finishing steel sheet. The hole expansion rate is ≥30%, which is small and does not meet the processing requirements of high-formability automotive parts. In addition, the process is prepared by hot rolling + pickling + cold rolling + continuous annealing + finishing, which has a large process flow and high production cost. In particular, pickling is required, which causes a lot of acid mist pollution, pollutes the environment and has high waste acid treatment costs.

[0004] Chinese patent application CN 104561775A discloses an economical high-strength hot-rolled galvanized steel sheet and its manufacturing method. It uses a common C-Mn composition system with the addition of a certain amount of V, Ti and Nb to produce a hot-rolled + pickled + galvanized steel sheet with an elongation of ≥30% and no requirement for hole expansion rate. However, it does not meet the processing requirements of high-formability automotive parts and requires pickling. The pickling process has significant acid mist pollution, which pollutes the environment and has high waste acid treatment costs. Summary of the Invention

[0005] To address the evolving needs of the automotive steel industry, this invention presents a high-formability, pickle-free hot-dip galvanized automotive steel and its production method. The steel sheet exhibits a yield strength ≥308 MPa, tensile strength ≥448 MPa, transverse elongation A ≥40%, hole expansion rate ≥121%, and meets the acceptable transverse cold bending standard (D=a at 180°). It also boasts excellent surface quality, free of color differences, and a surface roughness Ra of 0.7-1.4 μm. Furthermore, it eliminates the need for conventional hot rolling + pickling + cold rolling + continuous annealing galvanizing + finishing processes. Instead, it utilizes a hot rolling + reduction descaling + continuous annealing galvanizing + finishing process, reducing the pickling and cold rolling steps, resulting in lower production costs and no environmental pollution.

[0006] To achieve the above objectives, the present invention employs the following technical solution: A high-formability, acid-free hot-dip galvanized automotive steel, wherein the chemical composition of the steel by weight percentage is: C: 0.040%–0.070%, Si: 0.30%–0.42%, Mn: 1.35%–1.50%, Al: 0.035%–0.055%, Ti: 0.020%–0.050%, V: 0.010%–0.035%, Ge: 0.0035%–0.0085%, Pr: 0.006%–0.011%, with P ≤ 0.012%, S ≤ 0.005%, N ≤ 0.006%, and the balance being Fe and unavoidable impurities.

[0007] The finished steel plate has a ferrite volume percentage of 79%–88%, a bainite volume percentage of 10%–19%, and a pearlite volume percentage of 0%–2%.

[0008] The finished steel plate has a yield strength ≥308MPa, tensile strength ≥448MPa, transverse elongation A ≥40%, hole expansion rate ≥121%, transverse cold bending 180° D=a qualified, surface roughness Ra is 0.7~1.4μm, and the thickness of the finished steel plate is 1.50~2.25mm.

[0009] The main function of the high formability, pickling-free hot-dip galvanized automotive steel composition in this invention is: C: Carbon is a core strengthening element in steel. As an interstitial solid solution atom, it distorts the matrix lattice, achieving solid solution strengthening. In this invention, the key role of carbon is to ensure that the bainite content meets the standard, thereby simultaneously improving the strength, formability, and hole-expanding performance of the steel plate. If the carbon content is too low, the steel plate cannot meet the mechanical performance indicators required by this invention; if the content is too high, it will lead to embrittlement of the steel plate, causing delayed fracture and hot-rolling edge cracking, while significantly deteriorating its weldability and toughness. Therefore, this invention controls the carbon content within a low-carbon range, which can avoid the risks of delayed fracture and hot-rolling edge cracking, and also improve the weldability of the steel plate. Therefore, the optimal range of carbon in this invention is 0.040-0.070%.

[0010] Si: Silicon is the core element of this invention. Sufficient silicon addition to ferrite ensures the strength of the ferrite matrix and increases the AC3 temperature of the steel plate, widening the continuous annealing process window and ensuring that the ratio of ferrite to austenite in the critical region is within a reasonable range at industrial continuous annealing temperatures. Simultaneously, sufficient silicon content reduces inclusions in the steel and inhibits their formation, preventing inclusions from causing a decline in the mechanical properties and hole-expanding performance of the steel plate. However, the silicon content must be controlled within a reasonable range: too low a content will fail to guarantee matrix strength and achieve the inclusion inhibition effect, while too high a content will damage the surface quality of hot-rolled steel, causing a large amount of iron oxide scale to form and deteriorating the weldability of the steel plate. Therefore, the silicon content in this invention is 0.30-0.42%.

[0011] Mn: Manganese introduces lattice distortion into steel through substitutional solid solution, achieving solid solution strengthening. Simultaneously, as an austenite stabilizing element, it can widen the austenite region, reduce the critical quenching rate, and delay the transformation of austenite to pearlite. However, insufficient manganese content reduces the stability of supercooled austenite, leading to decreased plasticity and toughness of the steel plate. Manganese can dissolve in both ferrite and austenite phases, significantly improving the strength and hardness of both phases. The manganese content must be strictly controlled within a set range; excessive amounts can easily induce carbon or manganese segregation, disrupting the uniformity of the microstructure during hot rolling and inducing severe banded defects. It can also excessively increase the hardenability of the steel, inhibit bainite formation, impair the hole-expanding performance of the steel plate, and significantly degrade weldability. Therefore, considering all factors, this invention selects a manganese content of 1.35-1.50%.

[0012] P: Phosphorus is a harmful impurity in steel and tends to agglomerate at grain boundaries. When the phosphorus content in steel exceeds the standard, Fe2P particles will be generated, causing a decrease in the plasticity, toughness, and formability of the steel. Therefore, the phosphorus content must be strictly controlled, and the lower the content, the better it is for improving the formability of the steel. Thus, the upper limit is set at 0.012%.

[0013] S: Sulfur is a harmful impurity in steel. It easily reacts with manganese to form MnS inclusions. These inclusions become the core source of crack initiation, which not only deteriorates the processing performance of steel, but also significantly reduces the plasticity, formability and hole expansion performance of steel plates. Therefore, the lower the content, the better. The upper limit is set at 0.005%.

[0014] Al: In traditional steelmaking processes, Al is a deoxidizer. Al can also combine with nitrogen in steel to form AlN, refining the grain size. Simultaneously, together with Si, it inhibits cementite precipitation, increases the austenitizing temperature, facilitates better selection of the process window, and accelerates bainite transformation. Excessive Al content will cause nozzle blockage during continuous casting, affecting production efficiency and increasing production costs. Therefore, in this invention, the Al content is limited to 0.035-0.055%.

[0015] 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 ≤0.006% 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 and fatigue properties, thereby improving the service life of steel in the manufacture of automotive structural parts. Therefore, the optimal range of Ti content in this invention is between 0.020% and 0.050%.

[0016] Vanadium (V) has significant dual effects of precipitation strengthening and grain refinement strengthening. Its effect is mainly achieved by forming precipitates with carbon and nitrogen, especially the VN precipitate formed by combining with nitrogen, which can greatly improve the strength of the steel plate. At the same time, the large number of vanadium precipitates remaining in the steel plate structure can act as hydrogen traps, reducing the risk of delayed cracking during the service stage of the steel plate. In addition, the addition of vanadium promotes the formation of a large number of vanadium carbides in the steel, which also act as hydrogen traps, significantly enhancing the steel plate's resistance to hydrogen-induced cracking during service. Ultimately, the steel plate has excellent mechanical properties, hole expansion properties, and resistance to hydrogen-induced cracking. However, excessive vanadium content will lead to a decrease in the toughness of the weld heat-affected zone. Therefore, the amount of V added in this invention is 0.010-0.035%.

[0017] Ge (Ge): Germanium can improve the hot working properties of steel, reduce the tendency of steel to crack during rolling, and also optimize the weldability of steel, reducing the brittleness of weld joints and improving the consistency of the mechanical properties of weld joints. Furthermore, it can optimize the adhesion of the zinc layer, inhibit excessive thickness of the Fe-Zn brittle alloy layer, reduce coating peeling, flaking, and zinc loss during stamping and bending. It can also refine the coating structure, reduce porosity, make the zinc layer more uniform and dense, improve corrosion resistance, and improve the surface condition of the substrate, reduce oxidation defects, improve the wettability of zinc liquid, avoid defects such as incomplete plating and pinholes, and ensure the protective effect and appearance smoothness of the coating. Therefore, the optimal range of Ge content in this invention is between 0.0035% and 0.0085%.

[0018] Praseodymium (Pr) can remove harmful impurities such as O and S from steel, spheroidize brittle inclusions in the form of strips or angular shapes, reduce interface defects caused by inclusions during hot-dip galvanizing, improve zinc layer adhesion and stamping crack resistance, reduce the risk of coating peeling, refine substrate grains and improve surface oxide layer condition, inhibit abnormal thickening of Fe-Zn intermetallic compound layer during hot-dip galvanizing, promote coating microstructure refinement, reduce porosity and zinc flower size, and increase resistance to atmospheric corrosion and pitting corrosion. Therefore, the Pr content in this invention is limited to 0.006-0.011%.

[0019] 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. However, this invention requires the precipitation of TiN formed with Ti to carry out precipitation strengthening and grain refinement strengthening, thereby improving the strength of the steel plate. Therefore, the N content in this invention is ≤0.006%.

[0020] A method for producing high-formability, acid-free hot-dip galvanized automotive steel includes smelting, hot rolling, reduction descaling, continuous annealing galvanizing, and finishing. Specific methods include: The present invention uses the above chemical composition to produce steel plates through smelting, hot rolling, reduction descaling, continuous annealing galvanizing, and finishing. The specific manufacturing method includes the following steps: (1) Smelting process: The RH+LF process is adopted, and the H and O contents are strictly controlled. H≤0.0002% and O≤0.0015%. Calcium treatment is carried out in the refining process. Electromagnetic stirring and light reduction technology are applied in the continuous casting process. The billet casting speed is ≤2.2m / min and the light reduction amount is 2.5mm~5.0mm. This reduces the center segregation of the continuous casting billet, which is beneficial to reducing the banded structure in the subsequent hot-rolled steel plate.

[0021] (2) Hot rolling process: The continuous casting slab with a thickness of 150-230 mm and a width of 1050-2010 mm is directly hot-loaded into the walking beam furnace for heating at a temperature of 1200-1230℃ and a holding time of 130-180 min. The roughing process employs 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 roughing mill exit temperature is 1045℃~1075℃, the intermediate slab thickness is 35.0~50.0mm, and the width is 1050~2010mm. The intermediate slab is insulated with a heat shield before entering the hot finishing mill to reduce temperature drop on the delay roller table and temperature differences at the beginning and end and across the slab width. The finishing mill is a 7-stand continuous rolling process. High-pressure water descaling is performed before finishing, with the finishing mill inlet temperature not exceeding 1045℃. The final rolling temperature is 878~928℃, followed by laminar flow cooling at a rate of approximately 15~20℃ / s. After cooling to 626~658℃, the slab is coiled and air-cooled to room temperature. The rolled thickness is 1.50~2.25mm.

[0022] (3) Reduction Descaling: After cooling the hot-rolled steel coil with a thickness of 1.50-2.25mm to room temperature, the iron oxide scale on the surface of the steel plate is reduced and descaled in a high-temperature continuous heating furnace. The reduction descaling principle is as follows: Formula 1-6. The main processes include uncoiling, heating reduction, surface treatment, cooling, leveling and coiling. The heating reduction gas for the steel plate is a mixture of 60%-75% H2, 0%-15% CO and 15%-40% N2. The heating temperature is 630-1100℃. The surface treatment uses a heat-resistant steel wire roller brush to clean the reduced iron filings on the surface of the steel plate. The cooling gas is 100% N2. The cooling temperature is ≤65℃. The leveling elongation is 1.0-1.5%. After reduction descaling, the volume percentage of ferrite is 74%-85%, the volume percentage of lamellar pearlite is 3%-21%, and the volume percentage of spheroidized pearlite is 0%-23%.

[0023] Formula 1: Fe2O3+3H2→2Fe+3H2O; Formula 2: Fe3O4+4H2→3Fe+4H2O; Formula 3: FeO+H2→Fe+H2O; Formula 4: Fe2O3+3CO→2Fe+3CO2; Formula 5: Fe3O4+4CO→3Fe+4CO2; Formula 6: FeO+CO→Fe+CO2.

[0024] (4) Continuous annealing and galvanizing: The steel coil with a thickness of 1.50 to 2.25 mm after reduction and descaling is continuously annealed and galvanized. The strip speed is controlled at 60 to 110 m / min, the temperature of the soaking section is 795 to 815℃, the soaking time is 7 to 14 min, the slow cooling outlet temperature is 660 to 700℃, the rapid cooling rate is 27 to 32℃ / s, and the rapid cooling outlet temperature is 455 to 470℃. Then the strip is put into the zinc pot for hot galvanizing. The zinc pot temperature is 450 to 460℃ and the galvanizing time is 1 to 6 seconds. The soaking temperature is 795–815℃ to ensure a suitable ratio of ferrite to austenite in the critical zone. If the soaking temperature is higher than 815℃, the proportion of ferrite in the microstructure will decrease significantly, reducing the plasticity of the steel. If the soaking temperature is lower than 795℃, the austenite content in the critical zone will be significantly insufficient, affecting the subsequent bainite content and leading to insufficient strength. The soaking time is 7–14 minutes to ensure sufficient recrystallization of the steel plate grains during the heating and holding stages and to avoid excessive time leading to grain growth. If the soaking time is too short, the steel plate will not have enough time for continuous annealing and recrystallization, resulting in a decrease in the elongation of the steel plate. The slow cooling outlet temperature is 660-700℃, which aims to promote the rapid precipitation of ferrite. This inhibits grain growth while ensuring the ferrite content, thereby refining the ferrite grains. The fast cooling outlet temperature is 455-470℃, which aims to promote the rapid precipitation of bainite before the strip enters the zinc pot. This inhibits grain growth while ensuring the bainite content, thereby refining the bainite grains.

[0025] (5) Finishing: After galvanizing, the product enters the finishing machine, and the finishing elongation is 1.0% to 1.5%.

[0026] Compared with the prior art, the beneficial effects of the present invention are: 1. 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 ≤0.006% 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 and fatigue properties, thereby improving the service life of steel in automotive structural components.

[0027] 2. Vanadium has significant dual effects of precipitation strengthening and grain refinement strengthening. Its effect is mainly achieved by forming precipitates with carbon and nitrogen. In particular, the VN precipitate formed by combining with nitrogen can greatly improve the strength of steel plates. At the same time, the large number of vanadium precipitates remaining in the steel plate structure can act as hydrogen traps, reducing the risk of delayed cracking during the service stage of the steel plate. In addition, the addition of vanadium promotes the formation of a large number of vanadium carbides in the steel, which also act as hydrogen traps, significantly enhancing the steel plate's resistance to hydrogen-induced cracking during service. Ultimately, the steel plate has excellent mechanical properties, hole expansion properties and resistance to hydrogen-induced cracking.

[0028] 3. Germanium can improve the hot working properties of steel, reduce the tendency of steel to crack during rolling, and optimize the weldability of steel, reducing the brittleness of weld joints and improving the consistency of the mechanical properties of weld joints. It can also optimize the adhesion of the zinc layer, inhibiting excessive thickness of the Fe-Zn brittle alloy layer, reducing coating peeling, flaking, and zinc loss during stamping and bending. Furthermore, it can refine the coating structure, reduce porosity, making the zinc layer more uniform and dense, improving corrosion resistance, and improving the surface condition of the substrate, reducing oxidation defects, improving the wettability of the zinc bath, avoiding defects such as incomplete plating and pinholes, and ensuring the protective effect and smoothness of the coating.

[0029] 4. Praseodymium can remove harmful impurities such as O and S from steel, spheroidize brittle inclusions in the form of strips or angular shapes, reduce interface defects caused by inclusions during hot-dip galvanizing, improve zinc layer adhesion and stamping crack resistance, reduce the risk of coating peeling, refine substrate grains and improve the surface oxide layer state, inhibit abnormal thickening of Fe-Zn intermetallic compound layer during hot-dip galvanizing, promote coating microstructure refinement, reduce porosity and zinc flower size, and increase resistance to atmospheric corrosion and pitting corrosion.

[0030] 5. The microstructure of the steel of this invention consists of ferrite, bainite and pearlite, which significantly improves the comprehensive mechanical properties of the steel plate during the forming process.

[0031] 6. It does not require the conventional hot rolling + pickling + cold rolling + continuous annealing galvanizing + finishing process. Instead, it is obtained by hot rolling + reduction descaling + continuous annealing galvanizing + finishing process, which reduces the pickling + cold rolling process, resulting in low production cost and no environmental pollution. 7. The present invention has excellent mechanical properties, with yield strength ≥308MPa, tensile strength ≥448MPa, transverse elongation A ≥40%, hole expansion rate A ≥118%, transverse cold bending 180° D=a qualified, and surface roughness Ra is 0.7-1.4μm. Detailed Implementation

[0032] 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.

[0033] The smelting process employs RH+LF, with strict control over H and O content: H ≤ 0.0002%, O ≤ 0.0015%. Calcium treatment is performed during the refining process. Electromagnetic stirring and light reduction techniques are used during continuous casting, with a billet casting speed ≤ 2.2 m / min and a light reduction of 2.5 mm-5.0 mm. Continuously cast slabs with a thickness of 150–230 mm and a width of 1050–2010 mm are directly hot-charged into a walking beam furnace for heating at 1200–1230℃ for 130–180 min. The roughing process employs 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 roughing mill exit temperature is 1045℃~1075℃, the intermediate slab thickness is 35.0~50.0mm, and the width is 1050~2010mm. The intermediate slab is insulated with a heat shield before entering the hot finishing mill to reduce temperature drop on the delay roller table and temperature differences at the beginning and end and across the slab width. The finishing mill is a 7-stand continuous rolling process. High-pressure water descaling is performed before finishing, with the finishing mill inlet temperature not exceeding 1045℃. The final rolling temperature is 878~928℃, followed by laminar flow cooling at a rate of approximately 15~20℃ / s. After cooling to 626~658℃, the slab is coiled and air-cooled to room temperature. The rolled thickness is 1.50~2.25mm. After cooling steel coils with a thickness of 1.50–2.25 mm to room temperature, the iron oxide scale on the steel plate surface is descaled in a high-temperature continuous heating furnace. The descaling principle is as shown in Formula 1-6. The main processes are uncoiling, heating and reduction, surface treatment, cooling, leveling, and coiling. The heating and reducing gas for the steel plate is a mixture of 60%–75% H2, 0%–15% CO, and 15%–40% N2, with a heating temperature of 630–1100℃. The surface treatment uses a heat-resistant steel wire brush to clean the reduced iron filings. The cooling gas is 100% N2, with a cooling temperature ≤65℃. The leveling elongation is 1.0–1.5%. Steel coils with a thickness of 1.50–2.25 mm, after reduction and descaling treatment, are continuously annealed and galvanized. The strip speed is controlled at 60–110 m / min, the soaking temperature is 795–815℃, the soaking time is 7–14 min, the slow cooling exit temperature is 660–700℃, the rapid cooling rate is 27–32℃ / s, and the rapid cooling exit temperature is 455–470℃. The strip is then placed in a zinc bath for hot-dip galvanizing at a temperature of 450–460℃ for 1–6 seconds. After galvanizing, the strip enters a finishing machine with a finishing elongation of 1.0–1.5%. The finished product thickness is 1.50–2.25 mm.

[0034] The specific components, hot rolling process, reduction descaling process, continuous annealing galvanizing process, steel plate properties, and volume percentage of the six embodiments of the present invention are shown in Tables 1-6.

[0035] Table 1 Chemical composition (wt, %) of embodiments of the present invention

[0036] Table 2 Hot rolling process of embodiments of the present invention

[0037] Table 3. Reduction and descaling process of the present invention embodiments

[0038] Table 4 Continuous Annealing and Galvanizing Process Regulations of the Embodiments of the Present Invention

[0039] Table 5 Mechanical performance parameters of embodiments of the present invention

[0040] Table 6. Percentage of tissue volume in embodiments of the present invention .

Claims

1. A high-formability, pickling-free hot-dip galvanized automotive steel, characterized in that, The chemical composition of the steel, by weight percentage, is as follows: C: 0.040%–0.070%, Si: 0.30%–0.42%, Mn: 1.35%–1.50%, Al: 0.035%–0.055%, Ti: 0.020%–0.050%, V: 0.010%–0.035%, Ge: 0.0035%–0.0085%, Pr: 0.006%–0.011%, with P ≤ 0.012%, S ≤ 0.005%, N ≤ 0.006%, and the balance being Fe and unavoidable impurities.

2. The high formability, pickling-free hot-dip galvanized automotive steel according to claim 1, characterized in that, The finished steel plate has a ferrite volume percentage of 79%–88%, a bainite volume percentage of 10%–19%, and a pearlite volume percentage of 0%–2%.

3. The high formability, pickling-free hot-dip galvanized automotive steel according to claim 1, characterized in that, The finished steel plate has a yield strength ≥308MPa, tensile strength ≥448MPa, transverse elongation A ≥40%, hole expansion rate ≥121%, transverse cold bending 180° D=a qualified, surface roughness Ra is 0.7~1.4μm, and the thickness of the finished steel plate is 1.50~2.25mm.

4. A method for producing high-formability, pickling-free hot-dip galvanized automotive steel as described in any one of claims 1-3, characterized in that, The processes include smelting, hot rolling, reduction descaling, continuous annealing galvanizing, and finishing. Specific methods include: 1) Reduction Descaling: After the hot-rolled steel coil is cooled to room temperature, the iron oxide scale on the steel plate surface is reduced and descaled in a continuous heating furnace. The process includes uncoiling, heating reduction, surface treatment, cooling, leveling, and coiling. The heating reduction gas for the steel plate is a mixture of 60%–75% H2, 0%–15% CO, and 15%–40% N2, with a heating temperature of 630–1100℃. The surface treatment uses a heat-resistant steel wire brush to remove the reduced iron filings from the steel plate surface. The cooling gas is 100% N2, with a cooling temperature ≤65℃. The leveling elongation is 1.0–1.5%. 2) Continuous annealing and galvanizing: The reduced and descaled steel coils are continuously annealed and galvanized. The strip speed is controlled at 60-110 m / min, the temperature of the soaking zone is 795-815℃, the soaking time is 7-14 min, the slow cooling exit temperature is 660-700℃, the rapid cooling rate is 27-32℃ / s, and the rapid cooling exit temperature is 455-470℃. After that, the strip is put into the zinc pot for hot-dip galvanizing. The zinc pot temperature is 450-460℃, and the galvanizing time is 1-6 s.

5. The production method of high-formability, acid-free hot-dip galvanized automotive steel according to claim 4, characterized in that, After reduction and descaling, the volume percentage of ferrite in the steel plate is 74%–85%, the volume percentage of lamellar pearlite is 3%–21%, and the volume percentage of spheroidized pearlite is 0%–23%.

6. The production method of high-formability, acid-free hot-dip galvanized automotive steel according to claim 4, characterized in that, The thickness of the hot-rolled steel coil is 1.50 to 2.25 mm.

7. The production method of high-formability, acid-free hot-dip galvanized automotive steel according to claim 4, characterized in that, The smelting process includes: using the RH+LF process, incorporating electromagnetic stirring and light reduction technology during continuous casting, with a billet casting speed ≤2.2m / min and a light reduction amount of 2.5mm~5.0mm.

8. The method for producing high-formability, acid-free hot-dip galvanized automotive steel according to claim 4, characterized in that, The hot rolling process includes: hot-charging the continuously cast slab into a heating furnace for heating at a temperature of 1200–1230°C for 130–180 min; the roughing mill exit temperature is 1045°C–1075°C; the intermediate slab is kept warm by an insulation cover before entering the hot rolling mill finishing mill; the finishing mill inlet temperature is not higher than 1045°C; the final rolling temperature is 878–928°C; after final rolling, laminar flow cooling is adopted at a rate of approximately 15–20°C / s; the slab is cooled to 626–658°C and then coiled and air-cooled to room temperature.

9. The method for producing high-formability, acid-free hot-dip galvanized automotive steel according to claim 8, characterized in that, The continuous casting slab has a thickness of 150-230 mm and a width of 1050-2010 mm, while the intermediate slab has a thickness of 35.0-50.0 mm and a width of 1050-2010 mm.

10. The method for producing high-formability, acid-free, hot-dip galvanized automotive steel according to claim 4, characterized in that, The smoothing elongation is between 1.0% and 1.5%.

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