800mpa grade hot-rolled strip steel for automobile beam with high surface quality and production method thereof

Abstract

This invention relates to the technical field of automotive frame steel, specifically to a hot-rolled strip steel for automotive frame steel with a high surface quality of 800MPa and its production method. Its chemical composition and weight percentage content are as follows: C: 0.05%–0.08%, Si: 0.06%–0.12%, Mn: 1.50%–1.70%, P≤0.015%, S≤0.002%, Als: 0.035%–0.055%, Nb: 0.030%–0.050%, Ti: 0.11%–0.13%, N≤0.0040%, with the remainder being Fe and unavoidable impurities. The resulting hot-rolled strip steel has a yield strength ≥750MPa, tensile strength ≥800MPa, elongation ≥17%, no cracking after 180° cold bending, a dense oxide layer of ≤10μm formed on the steel surface, and no iron filings or dust on the surface after roll forming, meeting the requirements for direct coating without shot blasting for automotive frame steel.

Description

Technical Field

[0001] This invention relates to the technical field of automotive frame steel, specifically to a hot-rolled strip steel for automotive frame steel with high surface quality of 800MPa grade and its production method. Background Technology

[0002] With the rapid development of the automotive industry and increasingly stringent environmental regulations, the automotive sector has placed higher demands on material performance and quality. Among these, the automotive frame, as a key component that bears the vehicle body structure, absorbs collision energy, and ensures driving safety, requires steel materials that not only possess high strength to meet the needs of vehicle weight reduction and structural safety, but also good plasticity, formability, sheet stability, and surface quality to adapt to complex roll forming and subsequent painting processes. Market demand for automotive frame steel that combines high strength, excellent sheet shape, and high surface quality continues to grow.

[0003] The application of high-strength automotive beam steel can significantly achieve vehicle lightweighting. However, in the actual production process, hot-rolled strip steel undergoes multiple processes such as heating, rolling, cooling, and coiling, easily leading to the formation of iron oxide scale on the strip surface. The structure and density of this scale directly affect the surface quality and subsequent processing results. Especially for beams formed by roll forming, a "staining gray" phenomenon often occurs on the surface, where a gray or powdery residue remains on the steel plate after the oxide layer peels off. Downstream users typically require additional shot blasting to improve the surface condition. The structure and composition of the iron oxide scale are influenced by multiple factors, including hot rolling process parameters, operating conditions, and the chemical composition of the steel. Simultaneously, automotive beam steel must meet three core indicators: high strength, good sheet shape, and high surface quality. However, in existing technologies, these three often conflict with each other. Therefore, how to achieve high surface quality while ensuring high strength (800MPa grade) and good sheet shape has become a pressing technical bottleneck for the industry.

[0004] Chinese patent CN108262357A discloses a method for preventing the powdery spalling of iron oxide scale on hot-rolled, pickled-free automotive beam steel. This method adjusts the iron oxide scale composition by controlling the Si content (0.28%–0.30%) and adding Cr (0.200%–0.225%) (increasing the Fe3O4 content to approximately 80% and decreasing the FeO content to approximately 4%), while simultaneously using a high-power fan (40,000 m³ / h airflow, 255 Pa air pressure) to accelerate cooling. While this method effectively suppresses the powdery spalling of iron oxide scale, the high Si and Cr content increases alloy costs and easily produces red iron oxide scale. Furthermore, the high-power fan significantly increases production costs, hindering large-scale application.

[0005] Chinese patent CN102319742A discloses a temperature control method for solving the black ash problem on the surface of hot-rolled automotive beam steel plates. It improves the oxide scale composition by reducing the heating temperature (1200-1230℃), using iron scale suppression and water double descaling, increasing the final rolling temperature (900-920℃) and laminar flow cooling temperature (610-630℃). However, its target steel grade is 510L, a low-strength grade far below the 800MPa grade requirement, and it does not involve microalloying design and plate shape control strategies, which cannot meet the stringent requirements of high-end automotive beams for high strength and shot blasting-free coating.

[0006] Chinese patent CN109913750A discloses a high-strength thin steel plate with high surface quality and its preparation method. The chemical composition, by mass percentage, is: C: 0.02–0.09, Si: 0.03–0.09, Mn: 1.71–1.99, P≤0.0010, S≤0.0005, Cr: 0.10–0.20, Mo: 0.08–0.17, Nb+Ti+V≤0.20, H≤0.0002, with the balance being Fe and unavoidable impurities. Following the composition design, the process involves an oxygen top-and-bottom blowing converter, LF refining, RH refining, continuous casting, rolling, and heat treatment. The resulting thin steel plate has a minimum thickness of 3.98 mm, a yield strength between 751 and 892 MPa, and a surface black ash content of less than 10 g / m². 2 However, it uses high levels of precious alloying elements such as Cr (0.10%–0.20%) and Mo (0.08%–0.17%), resulting in high raw material costs. At the same time, the introduction of Cr may still lead to the formation of red iron oxide scale under certain working conditions, which is not conducive to obtaining high surface quality in a stable manner.

[0007] In summary, there is currently no systematic production technology for hot-rolled strip steel for automotive beams with high surface quality (800MPa grade). There is an urgent need to develop a low-cost production process that combines high strength, good shape, and high surface quality to achieve direct coating without shot peening, thereby promoting the application and upgrading of high-end automotive beam steel. Summary of the Invention

[0008] To address the aforementioned technical problems, this invention provides a hot-rolled strip steel for automotive beams with a high surface quality of 800MPa and its production method. This hot-rolled strip steel employs a low-carbon, low-silicon-titanium-niobium composite microalloying low-cost composition design and appropriate hot-rolling process control. It exhibits a yield strength ≥750MPa, tensile strength ≥800MPa, excellent strip shape, a dense oxide layer ≤10μm on the surface, and no iron filings or dust after rolling.

[0009] The technical solution of this invention is as follows: In a first aspect, the present invention provides a hot-rolled strip steel for automobile beams with a high surface quality of 800MPa grade. The chemical composition and weight percentage content of the hot-rolled strip steel are as follows: C: 0.05%~0.08%, Si: 0.06%~0.12%, Mn: 1.50%~1.70%, P≤0.015%, S≤0.002%, Als: 0.035%~0.055%, Nb: 0.030%~0.050%, Ti: 0.11%~0.13%, N≤0.0040%, with the remainder being Fe and unavoidable impurities.

[0010] Preferably, the chemical composition and mass percentage content of the hot-rolled strip steel are as follows: C: 0.053%, Si: 0.08%, Mn: 1.60%, P: 0.011%, S: 0.0009%, Als: 0.042%, Nb: 0.037%, Ti: 0.11%, N: 0.0032%, with the remainder being Fe and unavoidable impurities.

[0011] Preferably, the chemical composition and mass percentage content of the hot-rolled strip steel are as follows: C: 0.058%, Si: 0.10%, Mn: 1.65%, P: 0.010%, S: 0.0008%, Als: 0.045%, Nb: 0.045%, Ti: 0.12%, N: 0.0027%, with the remainder being Fe and unavoidable impurities.

[0012] Furthermore, the microstructure of hot-rolled strip steel is mainly composed of micron-sized polygonal ferrite and nano-sized precipitates, with a grain size ≥12.

[0013] Furthermore, the hot-rolled strip steel has a yield strength ≥750MPa, tensile strength ≥800MPa, elongation ≥17%, and no cracking after 180° cold bending; a dense oxide layer of ≤10μm is formed on the steel surface.

[0014] Furthermore, the thickness of the hot-rolled strip is 3.0–7.0 mm.

[0015] Secondly, the present invention provides a method for producing hot-rolled strip steel for automotive beams with a high surface strength of 800MPa grade, comprising the following steps: Step 1: Steelmaking and continuous casting. The chemical composition and weight percentage content are as follows: C: 0.05%~0.08%, Si: 0.06%~0.12%, Mn: 1.50%~1.70%, P≤0.015%, S≤0.002%, Als: 0.035%~0.055%, Nb: 0.030%~0.050%, Ti: 0.11%~0.13%, N≤0.0040%, with the remainder being Fe and unavoidable impurities. The molten steel is smelted according to the above composition requirements and continuously cast into slabs with a thickness of 228~232mm.

[0016] Step Two: Heating. The slab is heated in the furnace for 180–260 minutes, including a soaking period of 40–70 minutes. The slab exit temperature is 1235–1265℃. This heating regime ensures sufficient solid solution of microalloying elements such as titanium and niobium, while avoiding excessively high temperatures that could lead to austenite grain coarsening. The longer heating and soaking times help eliminate internal temperature differences within the slab, reducing the adverse effects of localized component segregation and temperature inhomogeneity on subsequent rolling stability and shape control. When the exit temperature is controlled at 1235–1265℃, it is beneficial to fully utilize the precipitation strengthening effect of the microalloying element titanium, while also reducing the difficulty of subsequent rolling and shape control.

[0017] Step 3: Rolling, including roughing and finishing rolling Rough rolling: A 1+5 rolling mode is adopted, first using a two-high roughing mill R1 for one pass, and then using a four-high roughing mill R2 for five passes to obtain an intermediate slab; the roughing speed is 3-4 m / s, and the exit temperature of the last pass is 1020-1060℃. The 1+5 rolling mode refines the austenite grains through rolling in the recrystallization zone, while gradually thinning the slab to the target intermediate slab thickness; a slow rolling speed reduction and water cooling strategy is adopted to effectively reduce the thickness of surface iron oxide scale and provide ideal entry conditions for subsequent finish rolling.

[0018] Finishing Rolling: The intermediate billet enters the seven-stand finishing mill for rolling. The entry temperature is 980–1010℃, the finishing speed is 7–10 m / s, and the finishing temperature is 860–900℃. The intermediate billet enters the seven-stand finishing mill at an entry temperature of 980–1010℃ to roll in the non-recrystallization zone to refine the austenite grains. The inhibitory effect of Nb on austenite recrystallization is fully utilized to achieve further grain refinement and strengthening. At the same time, a high-speed rolling strategy of 7–10 m / s is adopted to shorten the residence time of the strip in the high-temperature zone, reduce the deformation-induced precipitation of TiC in the finishing stage, and create conditions for the precipitation of more and finer TiC during the subsequent cooling and coiling processes.

[0019] Step 4: Cooling. A combination of pre-cooling and laminar flow cooling is employed, both using an intermittent cooling mode. The water cooling rate is 48℃ / s~55℃ / s, and the post-rolling air cooling time is ≤10s. The strip immediately enters the water cooling zone after finishing rolling. By combining pre-cooling and laminar flow cooling with an intermittent cooling mode, the cooling water is moved forward, minimizing the contact time between the strip and air while ensuring strip shape stability. The air cooling time is controlled within 10s, effectively reducing the formation of tertiary iron oxide scale. The cooling rate is controlled at 48~55℃ / s, allowing austenite to transform into fine and uniform ferrite under suitable conditions, while simultaneously promoting the dispersed precipitation of titanium carbides in the ferrite, avoiding grain growth and unfavorable microstructure formation.

[0020] Step 5: Coiling. The coiling temperature is 560℃~600℃, and the coiling tension is 16~18MPa. This coiling temperature range is conducive to the full precipitation of titanium carbides, thereby further improving the strip strength. Controlling the coiling tension at 16~18MPa can effectively suppress edge loosening and warping of the steel coil, while reducing oxidation caused by edge contact with air.

[0021] Furthermore, in step two, after the slab exits the furnace, it undergoes descaling in a high-pressure water descaling box with a descaling pressure ≥25MPa. The descaling process, performed in the high-pressure water descaling box after the slab exits the furnace, effectively removes the primary iron oxide scale formed during the high-temperature heating stage.

[0022] Furthermore, in step three, high-pressure water descaling is used both before and after the two-high roughing mill R1 in the roughing stage, with a descaling pressure ≥25MPa; descaling is activated in passes 1, 3, and 5 before the four-high roughing mill R2; descaling is activated before the finishing mill in the finishing stage, and medium-pressure descaling water is fully activated between stands F1 / F2 and F2 / F3, with a descaling pressure ≥10MPa. High-pressure water descaling is installed at key stand locations during roughing to promptly remove iron oxide scale generated during rolling, further reducing the risk of surface defects; medium- and high-pressure descaling water is installed before the finishing mill and between stands to remove secondary iron oxide scale remaining in the intermediate slabs, further improving the surface condition of the strip.

[0023] Furthermore, in step four, the number of water-cooling manifolds opened is adjusted according to the target thickness of the hot-rolled strip. When the target thickness of the hot-rolled strip is 3.0–5.0 mm, one water-cooling manifold is opened in each group; when the target thickness of the hot-rolled strip is 5.01–7.0 mm, two water-cooling manifolds are opened in each group. This ensures that the strip is cooled by water to the maximum extent possible while maintaining the quality of the strip shape, minimizing its contact time with air.

[0024] Furthermore, in step five, the coiled steel coils are stacked and slowly cooled for at least 48 hours. This slow cooling process, which involves stacking and slowly cooling the coils after coiling, requires no ventilation and lasts for at least 48 hours. Ventilation is prohibited during this time, allowing the steel coils to complete the microstructure stabilization process in a relatively enclosed, slowly cooling environment. This further suppresses edge oxidation and ultimately forms a dense oxide layer with a thickness of no more than 10 μm on the surface of the steel coil, ensuring that there are no iron oxide dust defects after roll forming.

[0025] Furthermore, in step three, the thickness of the intermediate billet is 36–42 mm.

[0026] The selection principles for each alloying element and its content in this invention are as follows: Carbon (C): C is one of the most economical and effective solid solution strengthening elements. It forms carbide precipitation strengthening with Ti and Nb, which is the basis for ensuring a strength of 800 MPa. However, the C content should not be too high. If the C content is >0.08%, it will impair the plasticity and toughness of the steel and easily lead to coarsening of carbide particles, thus losing the strengthening effect. Therefore, this invention controls the C content within the range of 0.05% to 0.08% to balance strength, plasticity, and manufacturing stability.

[0027] Silicon (Si): Si plays a role in solid solution strengthening. Adding an appropriate amount of Si can improve the elongation of steel. However, improper addition can easily lead to red iron scale defects on the surface, so it should be strictly limited. Therefore, this invention controls the Si content within the range of 0.06% to 0.12%.

[0028] Manganese (Mn): Mn is also a solid solution strengthening element. It can also expand the austenite region, stabilize austenite, lower the phase transformation temperature, and refine the grains. Excessive Mn content can easily lead to decreased plasticity, compositional segregation, and damage to the uniformity of steel structure. Therefore, in order to balance the strength and plasticity of steel, this invention controls the Mn content within the range of 1.50% to 1.70%.

[0029] Phosphorus (P): P is a harmful impurity element. Excessive P content can easily cause brittleness and impair ductility. Therefore, this invention controls the P content to within the range of ≤0.015%.

[0030] Sulfur (S): S is also a harmful impurity element, which easily forms plastic MnS inclusions, increases anisotropy, and seriously impairs ductility. Therefore, the present invention controls the S content within the range of ≤0.002%.

[0031] Aluminum (Al): Adding an appropriate amount of Al helps to inhibit the growth of iron oxide scale, reduce the thickness of the iron oxide scale, and improve density and bonding strength. However, excessive Al can easily form Al2O3 inclusions, which impairs toughness. Therefore, this invention controls the Al content within the range of 0.035% to 0.055%.

[0032] Niobium (Nb): The main role of Nb in steel is grain refinement and precipitation strengthening. By inhibiting austenite recrystallization and the precipitation of Nb(C,N), combined with controlled rolling and air cooling processes, grain refinement and precipitation strengthening can be achieved, improving the uniformity of the steel's microstructure while simultaneously increasing strength and toughness. If the Nb content is too low, the grain refinement and strengthening effects are not significant. If the Nb content is too high, the strengthening effect is not obvious, costs are increased, and it may impair the high-temperature plasticity of the cast billet and the stability of austenite. Therefore, based on a cost-performance balance, this invention controls the Nb content within the range of 0.030% to 0.050%.

[0033] Titanium (Ti): Ti is also a core strengthening element, possessing both nitrogen-fixing and precipitation strengthening functions. The formation of nano-sized TiN can suppress austenite grain growth during high-temperature heating; the precipitation of nano-sized TiC during coiling significantly improves strength. Simultaneously, its cost is relatively low, allowing it to replace some expensive alloys. If the Ti content is too low, the strengthening effect is not significant; if the Ti content is too high, excessive volume fraction TiC is easily formed and tends to coarsen, reducing ductility and toughness, rendering the strengthening effect ineffective, and TiN particles easily become crack sources during processing. Therefore, considering both strengthening and ductility, this invention controls the Ti content within the range of 0.11% to 0.13%.

[0034] Nitrogen (N): N is an impurity element, but when the N content is too high, it easily combines with Ti to form coarse and harmful micron-sized TiN, which becomes a source of processing cracks and also reduces the effective Ti content. Therefore, it must be strictly controlled. Therefore, this invention controls the N content within the range of ≤0.0040%.

[0035] The beneficial effects of this invention are as follows: (1) This invention adopts a low-cost composition design system of low C-appropriate Mn-Ti / Nb composite microalloying. By controlling the carbon content at a low level and rationally designing the manganese content, the solid solution strengthening and phase transformation strengthening effects are fully utilized. At the same time, the silicon content is strictly limited to reduce the adverse effects of silicon on the surface iron oxide scale structure, thereby improving the surface quality of the strip steel from the composition source. This invention does not add expensive alloying elements such as Mo and Cr. Instead, it achieves the superposition effect of fine grain strengthening and precipitation strengthening through the synergistic microalloying of titanium and niobium. Niobium improves the material strength and microstructure uniformity by inhibiting austenite recrystallization and participating in precipitation strengthening. Titanium inhibits grain growth in the form of TiN at high temperature and precipitates nanoscale TiC in the post-rolling cooling and coiling stages to further improve strength. Thus, it takes into account high strength, good plate shape and high surface quality, while achieving low-cost production.

[0036] (2) On the other hand, based on the balance of performance and shape, this invention achieves high strength and excellent shape of strip steel by rationally designing and precisely controlling the key process parameters of heating, rolling, cooling and coiling, and strengthening descaling. The hot-rolled strip steel has a yield strength ≥750MPa, tensile strength ≥800MPa, elongation ≥17%, and no cracking when cold-bent at 18°. At the same time, a dense oxide layer of ≤10μm is formed on the surface of the strip steel, which effectively avoids the "black and gray" defect after roll forming, meets the requirements of direct coating of automobile beams without shot blasting, and has the characteristics of low cost, simple process, excellent performance and applicability to mainstream thickness specifications (3~7mm). Attached Figure Description

[0037] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, for those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0038] Figure 1 This is a micrograph of the hot-rolled strip steel obtained in Example 4 of the present invention.

[0039] Figure 2 This is a photograph of the surface of the hot-rolled strip steel obtained in Example 4 of the present invention after being rolled.

[0040] Figure 3 These are photographs showing the morphology and thickness of the iron oxide scale on the surface of the hot-rolled strip steel obtained in Example 4 of this invention.

[0041] Figure 4 This is a micrograph of the hot-rolled strip steel obtained in Comparative Example 4 of this invention.

[0042] Figure 5 This is a photograph of the surface of the hot-rolled strip steel obtained in Comparative Example 4 of this invention after being rolled.

[0043] Figure 6 These are photographs showing the morphology and thickness of the iron oxide scale on the surface of the hot-rolled strip steel obtained in Comparative Example 4 of this invention. Detailed Implementation

[0044] To enable those skilled in the art to better understand the technical solutions of this invention, the technical solutions of the embodiments of this invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this invention, and not all embodiments. Based on the embodiments of this invention, all other embodiments obtained by those skilled in the art without creative effort should fall within the scope of protection of this invention.

[0045] This invention discloses a hot-rolled strip steel for automotive beams with a high surface quality of 800MPa grade. The chemical composition and weight percentage (wt%) are as follows: C: 0.05%~0.08%, Si: 0.06%~0.12%, Mn: 1.50%~1.70%, P≤0.015%, S≤0.002%, Als: 0.035%~0.055%, Nb: 0.030%~0.050%, Ti: 0.11%~0.13%, N≤0.0040%, with the remainder being Fe and unavoidable impurities.

[0046] The chemical composition of Examples 1-5 and Comparative Examples 1-5 of the present invention is shown in Table 1.

[0047] Table 1 Chemical composition of Examples 1-5 and Comparative Examples 1-5 of the present invention

[0048] The production method of the above-mentioned 800MPa grade high surface quality hot-rolled strip steel for automotive beams includes the following steps: Step 1: Steelmaking and continuous casting, using the following process: hot metal pretreatment → converter steelmaking → alloy fine-tuning station → LF ladle refining → RH ladle refining → continuous casting → billet heating → rolling → cooling → coiling preparation. After ladle refining, the chemical composition of the steel meets the following weight percentage (wt%): C: 0.05%~0.08%, Si: 0.06%~0.12%, Mn: 1.50%~1.70%, P≤0.015%, S≤0.002%, Als: 0.035%~0.055%, Nb: 0.030%~0.050%, Ti: 0.11%~0.13%, N≤0.0040%, with the remainder being Fe and unavoidable impurities. The molten steel is then smelted according to the above composition requirements and continuously cast into slabs with a thickness of 230mm.

[0049] Step 2: Heating. The slab is heated in the furnace for 180-260 minutes, of which the soaking time is 40-70 minutes. The slab exiting the furnace is 1235-1265℃. After exiting the furnace, it is descaled in a high-pressure (≥25MPa) water descaling box.

[0050] Step 3: Rolling, including roughing and finishing rolling Roughing: The 1+5 rolling mode is adopted. First, the slab is rolled in one pass using a two-high roughing mill R1, and then rolled in five passes using a four-high roughing mill R2 to gradually thin the slab to the target intermediate slab thickness to obtain the intermediate slab. High-pressure (≥25MPa) water descaling is carried out before and after R1. Descaling is started in the first, third and fifth passes before R2. The roughing speed is 3-4m / s, the exit temperature of the last pass of roughing is 1020-1060℃, and the thickness of the intermediate slab after roughing is 36mm.

[0051] Finishing rolling: The intermediate billet enters the seven-stand finishing mill for rolling. Before finishing rolling, the finishing descaling box is opened and high-pressure water is used. The finishing mill inlet temperature is 980~1010℃, the finishing mill speed is 7~10m / s, the finishing mill final rolling temperature is 860~900℃, and finally rolled into strip steel with a thickness of 3.0~7.0mm.

[0052] Step 4: Cooling. Immediately after rolling, the water is cooled. ≤4 laminar flow fine-tuning manifolds are opened to move the cooling water forward. Both the pre-cooling ultra-fast cooling and laminar flow cooling adopt an intermittent cooling mode. Among them, 1 water cooling manifold is always opened for each group of 3-5mm, and 2 water cooling manifolds are always opened for each group of 5.01-7mm. The water cooling rate is 48℃ / s~55℃ / s, and the air cooling time is ≤10s. Step 5: Coiling. The coiling temperature of the strip is set to 560℃~600℃, and the coiling tension is set to 16~18MPa. After coiling, the steel coil is immediately hoisted into the slow cooling zone for slow cooling. Ventilation is prohibited. The slow cooling time is ≥48h.

[0053] The main process parameters of Examples 1-5 and Comparative Examples 1-5 of the present invention are shown in Table 2.

[0054] Table 2. Main process parameters of Examples 1-5 and Comparative Examples 1-5

[0055] Performance testing: The hot-rolled strip steels obtained in Examples 1-5 and Comparative Examples 1-5 were tested for mechanical properties, 180° cold bending, surface quality, and strip shape quality. The test data are shown in Table 3.

[0056] Table 3. Test results of mechanical properties and surface quality of Examples 1-5 and Comparative Examples 1-5 of the present invention

[0057] (1) As shown in Table 3, Examples 1-5 of the present invention exhibit stable and excellent comprehensive performance under different thicknesses (3.0-7.0 mm). In terms of mechanical properties, the yield strength of Examples 1-5 is 767-801 MPa and the tensile strength is 821-836 MPa, which stably meet the strength requirements of 800 MPa grade automotive beam steel, and the fluctuation with thickness is small, indicating that the composition design and controlled rolling and cooling process of the present invention have good adaptability and stability. At the same time, the elongation of each example is not less than 19.0%, and no cracking occurred in the 180° cold bending test, indicating that while obtaining high strength, the steel still maintains good plasticity and formability. In terms of surface quality and plate shape quality, Examples 1-5 are all evaluated as "Pass" and "Good", and no iron gray, red iron sheet or obvious plate shape defects are found. This demonstrates that by controlling the low silicon content, multi-stage high-pressure descaling, and the synergistic effect of pre-cooling and reasonable coiling and slow cooling processes, a dense and stable thin oxide layer can be formed on the surface of the strip steel, effectively inhibiting the peeling of iron oxide scale and meeting the application requirements of direct coating without shot blasting.

[0058] (2) As shown in Table 3, the yield strength of Comparative Example 1 is 789 MPa, the tensile strength is 830 MPa, the elongation is 19.4%, and no cracking occurred after 180° cold bending. The mechanical properties are basically up to standard, but its strip shape quality is evaluated as "edge waviness". According to the process parameters in Table 2, the finishing rolling speed of this comparative example is too high (10.3 m / s), the water cooling rate is too high (64℃ / s), and the air cooling time is too short (6.7s), which leads to uneven cooling after rolling and residual stress concentration at the edge of the strip, thus producing obvious edge waviness defects.

[0059] Comparative Example 2 has a yield strength of 784 MPa, a tensile strength of 820 MPa, and an elongation of 18.5%, exhibiting acceptable cold bending performance, but its surface quality is rated as "Fail". According to the composition in Table 1, its Si content reaches 0.18%, significantly exceeding the range specified in this invention. Excessive Si content easily promotes Si enrichment and oxidation, generating loose and easily peeling iron oxide scale, thus leading to surface defects. Simultaneously, its coiling tension is only 12 MPa, which is not conducive to suppressing edge oxidation and scale transformation during the coiling and slow cooling stages.

[0060] Comparative Example 3 exhibits a yield strength and tensile strength of 804 MPa and 884 MPa, respectively, indicating a high strength level. However, its elongation is only 15.5%, and it cracked upon 180° cold bending, demonstrating significantly insufficient overall forming performance. The process parameters reveal a water cooling rate of only 37℃ / s and a high winding temperature of 623℃, leading to coarsening of the precipitated phase and uneven microstructure. While achieving high strength, this significantly sacrifices plasticity and toughness.

[0061] Comparative Example 4 has a yield strength of only 735 MPa and a tensile strength of 780 MPa, which does not meet the strength requirements for 800 MPa grade automotive beam steel. Although its elongation is relatively high (20.5%) and its cold bending performance is acceptable, its surface quality is rated as "Fail". Its low finishing rolling speed (6.1 m / s) and excessive air cooling time (11.4 s) caused the strip to stay in the high-temperature zone for too long, resulting in thickened iron oxide scale and decreased surface quality. At the same time, the low coiling temperature (535℃) also led to insufficient precipitation, which in turn weakened the strength.

[0062] Comparative Example 5 has a yield strength of 745 MPa and a tensile strength of 779 MPa. Although it has a relatively high elongation (21%) and acceptable surface quality, its strength level is significantly lower than the 800 MPa requirement. As shown in Table 2, its furnace exit temperature is too low (1210℃) and its water cooling rate is too low (41℃ / s), resulting in insufficient solid solution and precipitation strengthening effects of microalloying elements, and failing to fully realize the strengthening potential of Ti-Nb composite microalloying.

[0063] Micrographs of the hot-rolled strip steel obtained in Example 4 are shown below. Figure 1 As shown; a photograph of the surface of the hot-rolled strip obtained in Example 4 after roll forming is shown. Figure 2 As shown in the photographs; the morphology and thickness of the iron oxide scale on the surface of the hot-rolled strip steel obtained in Example 4 are shown in the photographs. Figure 3 As shown; a micrograph of the hot-rolled strip obtained in Comparative Example 4 is shown. Figure 4 As shown; a photograph of the surface of the hot-rolled strip obtained in Comparative Example 4 after roll forming is shown. Figure 5 As shown in the photographs; Comparative Example 4 shows the morphology and thickness of the surface oxide scale on the hot-rolled strip. Figure 6As shown. The microstructure of the hot-rolled strip steel obtained in Example 4 is ferritic, with a dense oxide layer of about 7.5 μm on the surface, which is well bonded to the matrix and does not fall off after roll forming, resulting in good surface quality of the steel. The microstructure of the hot-rolled strip steel obtained in Comparative Example 4 is a finer ferritic structure, with a loose oxide layer of about 13.5 μm on the surface, which is poorly bonded to the matrix and cracks and falls off after roll forming black oxide ash, resulting in poor surface quality of the steel.

[0064] In summary, Comparative Examples 1-5 exhibit significant shortcomings in terms of strip shape quality, surface quality, formability, and strength level, respectively. None of them achieved a synergistic effect of 800MPa-level high strength, excellent plasticity, good strip shape, and high surface quality within a thickness range of 3-7mm. This further demonstrates that the present invention, through low-carbon, low-silicon Ti / Nb composite microalloying composition design and synergistic optimization of heating, rolling, cooling, and coiling process parameters, achieves high strength and excellent strip shape in the strip. Simultaneously, a dense oxide layer of ≤10μm is formed on the strip surface, effectively avoiding the "black ash" defect after roll forming. This meets the requirements for direct shot-peening-free coating of automotive beams and features low cost, simple process, excellent performance, and applicability to mainstream thickness specifications (3-7mm).

[0065] As described above, although the present invention has been described in detail with reference to the accompanying drawings and preferred embodiments, the present invention is not limited thereto. Various equivalent modifications or substitutions can be made to the embodiments of the present invention by those skilled in the art without departing from the spirit and essence of the invention, and such modifications or substitutions should all be within the scope of the present invention. Any variations or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in the present invention should also be covered within the protection scope of the present invention.

Claims

1. A hot-rolled strip steel for automotive beams with a high surface finish of 800MPa, characterized in that, The chemical composition and weight percentage content of hot-rolled strip steel are as follows: C: 0.05%~0.08%, Si: 0.06%~0.12%, Mn: 1.50%~1.70%, P≤0.015%, S≤0.002%, Als: 0.035%~0.055%, Nb: 0.030%~0.050%, Ti: 0.11%~0.13%, N≤0.0040%, with the remainder being Fe and unavoidable impurities.

2. The hot-rolled strip steel for automotive beams with a surface strength of 800MPa as described in claim 1, characterized in that, The microstructure of hot-rolled strip steel is mainly composed of micron-sized polygonal ferrite and nano-sized precipitates, with a grain size ≥12.

3. The hot-rolled strip steel for automotive beams with a surface strength of 800MPa as described in claim 2, characterized in that, The hot-rolled strip steel has a yield strength ≥750MPa, tensile strength ≥800MPa, elongation ≥17%, and no cracking after 180° cold bending; a dense oxide layer of ≤10μm is formed on the steel surface.

4. The hot-rolled strip steel for automotive beams with a surface hardness of 800MPa as described in claim 1, characterized in that, The thickness of hot-rolled strip steel is 3.0 to 7.0 mm.

5. A method for producing 800MPa grade high-surface-weight hot-rolled strip steel for automotive beams as described in any one of claims 1 to 4, characterized in that, Includes the following steps: Step 1: Steelmaking and continuous casting. The chemical composition and mass percentage content are as follows: C: 0.05%~0.08%, Si: 0.06%~0.12%, Mn: 1.50%~1.70%, P≤0.015%, S≤0.002%, Als: 0.035%~0.055%, Nb: 0.030%~0.050%, Ti: 0.11%~0.13%, N≤0.0040%, with the remainder being Fe and unavoidable impurities. Steel is smelted according to the above composition requirements and continuously cast into slabs with a thickness of 228~232mm. Step 2: Heating. The slab is heated in the furnace for 180-260 minutes, of which the soaking time is 40-70 minutes. The slab exiting the furnace is 1235-1265℃. Step 3: Rolling, including roughing and finishing rolling Roughing: The 1+5 rolling mode is adopted. First, one pass of rolling is performed using a two-high roughing mill R1, and then five passes of reciprocating rolling are performed using a four-high roughing mill R2 to obtain an intermediate billet. The roughing speed is 3-4 m / s, and the exit temperature of the last pass of roughing is 1020-1060℃. Finishing rolling: The intermediate billet enters the seven-stand finishing mill for rolling. The finishing mill inlet temperature is 980-1010℃, the finishing mill speed is 7-10m / s, and the finishing mill finishing temperature is 860-900℃. Step 4: Cooling. Pre-cooling and laminar flow cooling are adopted. Both pre-cooling and laminar flow cooling adopt intermittent cooling mode. The water cooling rate is 48℃ / s~55℃ / s, and the air cooling time after rolling is ≤10s. Step 5: Winding, with a winding temperature of 560℃~600℃ and a winding tension of 16~18MPa.

6. The production method of hot-rolled strip steel for 800MPa grade high surface finish automotive beams as described in claim 5, characterized in that, In step two, after the slab exits the furnace, it is descaled by a high-pressure water descaling box with a descaling pressure ≥25MPa.

7. The production method of hot-rolled strip steel for 800MPa grade high surface finish automotive beams as described in claim 5, characterized in that, In step three, high-pressure water descaling is used both before and after the two-high roughing mill R1 in the roughing stage, with a descaling pressure ≥25MPa; descaling is started in passes 1, 3, and 5 before the four-high roughing mill R2; descaling is started before the finishing mill in the finishing stage, and medium-pressure descaling water is fully turned on between stands F1 / F2 and F2 / F3, with a descaling pressure ≥10MPa.

8. The production method of hot-rolled strip steel for 800MPa grade high surface finish automotive beams as described in claim 5, characterized in that, In step four, the number of water-cooled manifolds opened is adjusted according to the target thickness of the hot-rolled strip. When the target thickness of the hot-rolled strip is 3.0 to 5.0 mm, one water-cooled manifold is opened in each group. When the target thickness of the hot-rolled strip is 5.01 to 7.0 mm, two water-cooled manifolds are opened in each group.

9. The production method of hot-rolled strip steel for 800MPa grade high surface finish automotive beams as described in claim 5, characterized in that, In step five, the coiled steel is stacked and cooled slowly for ≥48 hours.

10. The method for producing 800MPa grade high-surface-weight hot-rolled strip steel for automotive beams as described in claim 5, characterized in that, In step three, the thickness of the intermediate billet is 36–42 mm.

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